spanmetrics: fall back to mapped attribute when no source_label matches

[buiducnhat]

What this PR does:

When a span-metrics dimension_mappings entry's name matches an attribute that is already present on a span/resource (e.g. http_response_status_code), that value is now used as a fallback when none of the configured source_labels match.

Motivation: With a config like

dimension_mappings:
  - name: http_response_status_code
    source_labels: [http.status_code]

spans that still send the legacy http.status_code get their value remapped to http_response_status_code, but spans that have already migrated to the new attribute end up with an empty http_response_status_code label and land on a different metric series. This splits a single logical signal across two series and makes migration of instrumentation conventions painful.

Behaviour change: only when no source_label produced a value does the processor consult the attribute named dimension_mappings.name directly. Existing behaviour for spans that match a source_label is unchanged (no double-counting, no reordering).

Which issue(s) this PR fixes:
Fixes #3256

Checklist

• Tests updated (TestSpanMetricsDimensionMappingIncludesMappedAttribute covers both the mapped and direct-attribute paths)
• Documentation added (not applicable — behaviour now matches the documented intent of dimension_mappings)
• Changelog entry added under .chloggen/

[github-actions]

Signed commits report

2 of 2 commits between main and fix/3256-spanmetrics-dimension-mapping-fallback could not be fully verified:

Commit	Author	Reason	Message
5abd4bcb	Nhat Bui	unsigned	spanmetrics: fall back to mapped attribute when no source_label matches
36fe1e9a	Nhat Bui	unsigned	chore: add chloggen entry for #3256

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