sdk/metric/exemplar: skip Offer on a zero-sized FixedSizeReservoir

[SAY-5]

Summary

NewFixedSizeReservoir already clamps k to 0 when a caller passes a negative value, but FixedSizeReservoir.Offer does not defend against r.k == 0. After a single call, count reaches next and the random-index branch runs:

idx := rand.IntN(int(r.k)) // rand.IntN(0) panics

which panics with invalid argument to IntN. The SDK doesn't expose this path today, but callers that hand-roll a reservoir (or reach it through a custom ReservoirProvider) can — exactly the case Copilot flagged on #8230 and tracked in this issue.

Fixes #8232.

Change

Short-circuit Offer when r.k == 0:

if r.k == 0 {
    return
}

Collect is already safe on an empty storage, so a zero-sized reservoir now silently drops every exemplar instead of panicking.

Testing

• go test ./sdk/metric/exemplar/... passes.
• Added a local Offer regression against a reservoir built with NewFixedSizeReservoir(0) and then called Offer 100 times; previously it panicked on the second call, now it runs cleanly and Collect returns an empty slice.

[dashpole]

Needs a unit test, and the giant comment probably isn't necessary. A one-line comment should be sufficient.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.7%. Comparing base (5bd5702) to head (6465356).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8243   +/-   ##
=====================================
  Coverage   82.6%   82.7%           
=====================================
  Files        310     310           
  Lines      24648   24650    +2     
=====================================
+ Hits       20381   20386    +5     
+ Misses      3890    3887    -3     
  Partials     377     377           

Files with missing lines	Coverage Δ
sdk/metric/exemplar/fixed_size_reservoir.go	89.4% <100.0%> (+0.3%)	⬆️

... and 2 files with indirect coverage changes

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[pellared]

Per #8250 (review)

[SAY-5]

@pellared sorry for the duplicate. To answer your questions directly:

• Yes I used Claude Code to help draft this; I read both diffs and validated each locally (running the Algorithm-L reset path with k=0 reproduces the +Inf and follow-on panic).
• The two PRs ended up tackling the same root cause from different angles — sdk/metric/exemplar: skip Offer on a zero-sized FixedSizeReservoir #8243 (this one) was a defensive guard inside Offer, sdk/metric/exemplar: gracefully handle FixedSizeReservoir of size 0 #8250 was a guard at reset so the +Inf is never seeded. The reset-side fix is the more fundamental of the two, so I'm closing this PR in favour of sdk/metric/exemplar: gracefully handle FixedSizeReservoir of size 0 #8250. Apologies again for the noise.