fix(metricstransformprocessor): preserve absent histogram sum on merge

[CedricConday]

Description

When merging (exponential) histogram datapoints in aggregateutil, the sum was set unconditionally (dp.SetSum(dp.Sum() + dps.At(i).Sum())). Because SetSum always marks the value present, merging datapoints that had no sum (HasSum() == false) produced HasSum() == true with Sum() == 0, which is indistinguishable from a genuine zero sum. Min and max in the same loop are already guarded with HasMin()/HasMax(); the sum is now handled the same way: it is only accumulated while every merged datapoint carries one, and is otherwise removed.

Link to tracking issue

Fixes #49379

Testing

Added a regression test (Test_MergeDataPoints_SumPresence) covering histogram and exponential-histogram merges for both the all-present case (sums add, HasSum stays true) and the any-absent case (HasSum becomes false). Existing Test_MergeDataPoints expectations that encoded the old behavior (a fabricated Sum=0 on sumless inputs) were corrected. The new and corrected tests fail before the change and pass after; go test, gofmt, and go vet pass for the package.

Documentation

No user-facing documentation changes; a .chloggen changelog entry is included.

Authorship

• I, a human, wrote this pull request description myself.

_{Authored with AI assistance (Claude Code) and reviewed before submission.}

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: CedricConday / name: Cedric Conday (b4695a8)

[github-actions]

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[singhvibhanshu]

/workflow-approve

[singhvibhanshu]

/workflow-approve