fix: off-by-one bug for FixedSizeReservoir

[dashpole]

While working on #8306, I found an off-by-one error.

Reset was setting r.next to k, and then calling advance(), which meant r.next was always at least k+1.

The first k exemplars are unconditionally stored during the "fill" phase. Count is incremented at the end of the Offer call, so the k+1th exemplar was never stored because r.next is k when that Offer call is being evaluated.

[Copilot]

Pull request overview

This PR fixes an off-by-one error in the FixedSizeReservoir (Algorithm L) reset initialization that previously made the first measurement after the reservoir filled (the k+1th offer) impossible to sample, and adds a regression test plus a changelog entry.

Changes:

• Fix FixedSizeReservoir.reset() initialization of r.next so the first post-fill replacement opportunity can occur at the correct count.
• Add a regression test that would deterministically fail under the prior behavior (second offered item never sampled for k=1).
• Document the fix in CHANGELOG.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
sdk/metric/exemplar/fixed_size_reservoir.go	Adjusts reset() to initialize r.next as cap-1 so the subsequent advance() produces the correct first eligible replacement count.
sdk/metric/exemplar/fixed_size_reservoir_test.go	Adds a regression test ensuring the k+1th offer can be sampled (verifies the off-by-one is fixed).
CHANGELOG.md	Adds a “Fixed” entry describing the off-by-one bug and its user-visible impact.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.9%. Comparing base (4086701) to head (74e40d7).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8309   +/-   ##
=====================================
  Coverage   82.9%   82.9%           
=====================================
  Files        314     314           
  Lines      24985   24985           
=====================================
+ Hits       20730   20733    +3     
+ Misses      3882    3880    -2     
+ Partials     373     372    -1     

Files with missing lines	Coverage Δ
sdk/metric/exemplar/fixed_size_reservoir.go	94.7% <100.0%> (ø)

... and 3 files with indirect coverage changes

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[dashpole]

Test failure is because this needs #8295