[cumulativetodeltaprocessor] Add histogram_fields config for selective field conversion

[Chau-Tran]

Description

Adds an optional histogram_fields allowlist to cumulativetodeltaprocessor. The list controls which histogram data-point fields (bucket_counts, sum, count) are converted to delta. When unset (default), all three fields are converted — identical to today's behavior.

When set, only the listed fields are written back from the tracker's delta value; the others keep their cumulative value on the emitted data point. The tracker itself still computes a full delta internally so state stays consistent across restarts, resets, and the existing bucket-mismatch handling — only the write-back to the data point is gated.

Applies to both Histogram and ExponentialHistogram. RemoveMin/RemoveMax continues to apply whenever any field converts (min/max can't be re-derived from a partial conversion). Reset / NaN / NoRecordedValue handling is unchanged.

processors:
  cumulativetodelta:
    include:
      metric_types: [histogram]
    histogram_fields: [bucket_counts]   # only buckets become delta; sum/count stay cumulative

Link to tracking issue

Fixes #48617

Testing

• go test -race ./... passes in processor/cumulativetodeltaprocessor and its sub-packages.
• make fmt lint passes (testifylint, gofmt, golangci-lint).
• make chlog-validate passes.
• New test cases (table-driven) cover: every single-field allowlist, every pairwise combination, all-three, the implicit empty-default, and a validation case for an unknown field name. Tests assert both that listed fields convert to delta and that unlisted fields retain their cumulative value across two data-point intervals.

Documentation

• README.md: new bullet under "Configuration" describing histogram_fields, and an example YAML block showing the prototypical bucket_counts-only opt-in.
• Changelog entry under .chloggen/.

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[TylerHelmuth]

@Chau-Tran please sign the CLA and confirm to the PR template's attestation (the I am a human checkbox)