feat(metrics): Add trace metric enrichment with default and user attributes (RUST-169)

[szokeasaurusrex]

Add metric enrichment in sentry-core so emitted trace metrics include:

• Trace/span association: trace_id from active span or propagation
  context, span_id from active span when not explicitly set.
• Default SDK attributes: sentry.environment, sentry.release,
  sentry.sdk.name, sentry.sdk.version, and server.address.
• User PII attributes (user.id, user.name, user.email) gated by
  send_default_pii.
• before_send_metric callback for filtering/modifying metrics.

Attribute merges use or_insert to preserve explicitly set metric
attributes, matching the behavior specified in
https://develop.sentry.dev/sdk/telemetry/metrics/#default-attributes
and https://develop.sentry.dev/sdk/telemetry/metrics/#user-attributes.

Scope and client enrichment logic is based on the approach from #997.

Co-authored-by: Joris Bayer joris.bayer@sentry.io

Closes #1024
Closes RUST-169

[linear-code]

RUST-169 Add trace metric default and user attribute enrichment in sentry-core

[github-actions]

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- Add trace metric enrichment with default and user attributes (RUST-169) ([#1031](https://github.com/getsentry/sentry-rust/pull/1031))

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