opentelemetry-sdk: fill every exemplar reservoir bucket before sampling

[henry3260]

Description

SimpleFixedSizeExemplarReservoir implements Algorithm R reservoir sampling, which
has two phases: the first size measurements fill each bucket directly, and every
measurement after that replaces a random bucket with probability size / seen.

_find_bucket_index incremented _measurements_seen before comparing it against
_size, so at the point of the check the counter meant "measurements seen including
this one" rather than "measurements seen before this one". The deterministic fill
phase therefore ran size - 1 times instead of size.

self._measurements_seen += 1
if self._measurements_seen < self._size:   # off by one
    return self._measurements_seen - 1

1. The uniform sampling guarantee no longer held. Algorithm R's correctness relies
   on the first size measurements always being retained. Losing one of them breaks the
   invariant that every measurement seen so far is retained with equal probability.

size=1 was unaffected (1 < 1 is false, and randrange(0, 1) is always 0).

The fix moves the increment after the comparison so the counter carries the meaning the
algorithm expects at the point of the check. No public API or configuration changes.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• New regression test — test_fills_every_bucket_before_sampling in
  opentelemetry-sdk/tests/metrics/test_exemplarreservoir.py. It offers exactly size
  measurements for size in (1, 2, 4, 10) and asserts that all size buckets are
  filled with exactly those measurements. The test fails on the current code
  (AssertionError: 9 != 10 for size=10) and passes with the fix.

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated