Improve explicit histogram contention performance

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/AbstractInstrument.java

@@ -5,22 +5,35 @@
 
 package io.opentelemetry.sdk.metrics;
 
+import io.opentelemetry.context.Context;
 import io.opentelemetry.sdk.metrics.internal.descriptor.InstrumentDescriptor;
 import javax.annotation.Nullable;
 
 abstract class AbstractInstrument {
 
   private final InstrumentDescriptor descriptor;
+  final boolean exemplarsAlwaysOff;
 
   // All arguments cannot be null because they are checked in the abstract builder classes.
-  AbstractInstrument(InstrumentDescriptor descriptor) {
+  AbstractInstrument(InstrumentDescriptor descriptor, SdkMeter sdkMeter) {
     this.descriptor = descriptor;
+    this.exemplarsAlwaysOff = sdkMeter.isExemplarsAlwaysOff();
   }
 
   final InstrumentDescriptor getDescriptor() {
     return descriptor;
   }
 
+  /**
+   * Returns {@link Context#current()}, or {@link Context#root()} when exemplars are known-off and
+   * the caller doesn't otherwise care about propagating a specific context. Used by parameterless
+   * record overloads on synchronous instruments to skip a thread-local lookup when the resulting
+   * context is only consulted by the exemplar path.
+   */
+  final Context currentOrRootContext() {
+    return exemplarsAlwaysOff ? Context.root() : Context.current();
+  }
+
   @Override
   public boolean equals(@Nullable Object o) {
     if (this == o) {

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/ExtendedSdkDoubleCounter.java

@@ -48,7 +48,7 @@ public BoundDoubleCounter bind(Attributes attributes) {
 
   @Override
   public void add(double value) {
-    add(value, Context.current());
+    add(value, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/ExtendedSdkDoubleGauge.java

@@ -47,7 +47,7 @@ public BoundDoubleGauge bind(Attributes attributes) {
 
   @Override
   public void set(double value) {
-    set(value, Context.current());
+    set(value, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/ExtendedSdkDoubleHistogram.java

@@ -48,7 +48,7 @@ public BoundDoubleHistogram bind(Attributes attributes) {
 
   @Override
   public void record(double value) {
-    record(value, Context.current());
+    record(value, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/ExtendedSdkDoubleUpDownCounter.java

@@ -48,7 +48,7 @@ public BoundDoubleUpDownCounter bind(Attributes attributes) {
 
   @Override
   public void add(double value) {
-    add(value, Context.current());
+    add(value, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/ExtendedSdkLongCounter.java

@@ -47,7 +47,7 @@ public BoundLongCounter bind(Attributes attributes) {
 
   @Override
   public void add(long value) {
-    add(value, Context.current());
+    add(value, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/ExtendedSdkLongGauge.java

@@ -46,7 +46,7 @@ public BoundLongGauge bind(Attributes attributes) {
 
   @Override
   public void set(long value) {
-    set(value, Context.current());
+    set(value, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/ExtendedSdkLongHistogram.java

@@ -48,7 +48,7 @@ public BoundLongHistogram bind(Attributes attributes) {
 
   @Override
   public void record(long value) {
-    record(value, Context.current());
+    record(value, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/ExtendedSdkLongUpDownCounter.java

@@ -48,7 +48,7 @@ public BoundLongUpDownCounter bind(Attributes attributes) {
 
   @Override
   public void add(long value) {
-    add(value, Context.current());
+    add(value, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkDoubleCounter.java

@@ -28,7 +28,7 @@ class SdkDoubleCounter extends AbstractInstrument implements DoubleCounter {
 
   SdkDoubleCounter(
       InstrumentDescriptor descriptor, SdkMeter sdkMeter, WriteableMetricStorage storage) {
-    super(descriptor);
+    super(descriptor, sdkMeter);
     this.sdkMeter = sdkMeter;
     this.storage = storage;
   }
@@ -48,7 +48,7 @@ public void add(double increment, Attributes attributes, Context context) {
 
   @Override
   public void add(double increment, Attributes attributes) {
-    add(increment, attributes, Context.current());
+    add(increment, attributes, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkDoubleGauge.java

@@ -23,7 +23,7 @@ class SdkDoubleGauge extends AbstractInstrument implements DoubleGauge {
 
   SdkDoubleGauge(
       InstrumentDescriptor descriptor, SdkMeter sdkMeter, WriteableMetricStorage storage) {
-    super(descriptor);
+    super(descriptor, sdkMeter);
     this.sdkMeter = sdkMeter;
     this.storage = storage;
   }
@@ -35,7 +35,7 @@ public boolean isEnabled() {
 
   @Override
   public void set(double value, Attributes attributes) {
-    storage.recordDouble(value, attributes, Context.current());
+    storage.recordDouble(value, attributes, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkDoubleHistogram.java

@@ -28,7 +28,7 @@ class SdkDoubleHistogram extends AbstractInstrument implements DoubleHistogram {
 
   SdkDoubleHistogram(
       InstrumentDescriptor descriptor, SdkMeter sdkMeter, WriteableMetricStorage storage) {
-    super(descriptor);
+    super(descriptor, sdkMeter);
     this.sdkMeter = sdkMeter;
     this.storage = storage;
   }
@@ -48,7 +48,7 @@ public void record(double value, Attributes attributes, Context context) {
 
   @Override
   public void record(double value, Attributes attributes) {
-    record(value, attributes, Context.current());
+    record(value, attributes, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkDoubleUpDownCounter.java

@@ -23,7 +23,7 @@ class SdkDoubleUpDownCounter extends AbstractInstrument implements DoubleUpDownC
 
   SdkDoubleUpDownCounter(
       InstrumentDescriptor descriptor, SdkMeter sdkMeter, WriteableMetricStorage storage) {
-    super(descriptor);
+    super(descriptor, sdkMeter);
     this.sdkMeter = sdkMeter;
     this.storage = storage;
   }
@@ -40,7 +40,7 @@ public void add(double increment, Attributes attributes, Context context) {
 
   @Override
   public void add(double increment, Attributes attributes) {
-    add(increment, attributes, Context.current());
+    add(increment, attributes, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkLongCounter.java

@@ -29,7 +29,7 @@ class SdkLongCounter extends AbstractInstrument implements LongCounter {
 
   SdkLongCounter(
       InstrumentDescriptor descriptor, SdkMeter sdkMeter, WriteableMetricStorage storage) {
-    super(descriptor);
+    super(descriptor, sdkMeter);
     this.sdkMeter = sdkMeter;
     this.storage = storage;
   }
@@ -49,7 +49,7 @@ public void add(long increment, Attributes attributes, Context context) {
 
   @Override
   public void add(long increment, Attributes attributes) {
-    add(increment, attributes, Context.current());
+    add(increment, attributes, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkLongGauge.java

@@ -22,7 +22,7 @@ class SdkLongGauge extends AbstractInstrument implements LongGauge {
   final WriteableMetricStorage storage;
 
   SdkLongGauge(InstrumentDescriptor descriptor, SdkMeter sdkMeter, WriteableMetricStorage storage) {
-    super(descriptor);
+    super(descriptor, sdkMeter);
     this.sdkMeter = sdkMeter;
     this.storage = storage;
   }
@@ -34,7 +34,7 @@ public boolean isEnabled() {
 
   @Override
   public void set(long value, Attributes attributes) {
-    storage.recordLong(value, attributes, Context.current());
+    storage.recordLong(value, attributes, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkLongHistogram.java

@@ -29,7 +29,7 @@ class SdkLongHistogram extends AbstractInstrument implements LongHistogram {
 
   SdkLongHistogram(
       InstrumentDescriptor descriptor, SdkMeter sdkMeter, WriteableMetricStorage storage) {
-    super(descriptor);
+    super(descriptor, sdkMeter);
     this.sdkMeter = sdkMeter;
     this.storage = storage;
   }
@@ -49,7 +49,7 @@ public void record(long value, Attributes attributes, Context context) {
 
   @Override
   public void record(long value, Attributes attributes) {
-    record(value, attributes, Context.current());
+    record(value, attributes, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkLongUpDownCounter.java

@@ -23,7 +23,7 @@ class SdkLongUpDownCounter extends AbstractInstrument implements LongUpDownCount
 
   SdkLongUpDownCounter(
       InstrumentDescriptor descriptor, SdkMeter sdkMeter, WriteableMetricStorage storage) {
-    super(descriptor);
+    super(descriptor, sdkMeter);
     this.sdkMeter = sdkMeter;
     this.storage = storage;
   }
@@ -40,7 +40,7 @@ public void add(long increment, Attributes attributes, Context context) {
 
   @Override
   public void add(long increment, Attributes attributes) {
-    add(increment, attributes, Context.current());
+    add(increment, attributes, currentOrRootContext());
   }
 
   @Override

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/SdkMeter.java

@@ -22,6 +22,7 @@
 import io.opentelemetry.sdk.metrics.data.MetricData;
 import io.opentelemetry.sdk.metrics.internal.MeterConfig;
 import io.opentelemetry.sdk.metrics.internal.descriptor.InstrumentDescriptor;
+import io.opentelemetry.sdk.metrics.internal.exemplar.AlwaysOffExemplarFilter;
 import io.opentelemetry.sdk.metrics.internal.export.RegisteredReader;
 import io.opentelemetry.sdk.metrics.internal.state.AsynchronousMetricStorage;
 import io.opentelemetry.sdk.metrics.internal.state.BoundStorageHandle;
@@ -88,6 +89,16 @@ final class SdkMeter implements Meter {
 
   private final MeterProviderSharedState meterProviderSharedState;
   private final InstrumentationScopeInfo instrumentationScopeInfo;
+
+  /**
+   * Returns true if the meter provider's exemplar filter samples nothing. Callers can use this to
+   * skip {@link io.opentelemetry.context.Context#current()} lookups on record paths that only need
+   * the current context to derive an exemplar span context.
+   */
+  boolean isExemplarsAlwaysOff() {
+    return meterProviderSharedState.getExemplarFilter() instanceof AlwaysOffExemplarFilter;
+  }
+
   private final Map<RegisteredReader, MetricStorageRegistry> readerStorageRegistries;
 
   private volatile boolean meterEnabled;