perf(codecs): reduce allocations in OTLP metric conversion

[bruceg]

Summary

Pass metric data by ownership through the OTLP metric encode paths to avoid redundant allocations:

• metric_event_to_export_request and split_metric_tags now consume the owned Metric/MetricTags, moving tag keys/values, metric name, buckets, and quantiles instead of cloning them (added MetricTags::into_iter_sets).
• build_metric_tags takes its attributes by value and moves keys/values.
• ResourceMetrics::into_event_iter (decode path) moves the metric name and drops the redundant per-metric resource/scope pre-clones.

Note that I did not include a changelog as the feature itself was noted in the changelogs and is not yet present in a release.

Vector configuration

N/A used #25923 to validate

How did you test this PR?

Running cargo bench --package codecs --features opentelemetry otlp/encode before and after the change shows an improvement in throughput of 10-52% depending on the metric data. Handling of labels/tags showed the largest improvements.

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Follow up to feat(codecs): support encoding native Vector metrics in OTLP serializer #25738

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[thomasqueirozb]

strip_prefix reads a little better here. Are you able to benchmark if the &str conversion makes any difference here?

[bruceg]

It appears to cause a repeatable ~4% regression with strip_prefix over this drain setup. Ideally we could use String::replace_first or String::remove_matches but those are still unstable and don't return if they made any changes.