Add more RequestMetadata attributes to trace spans

[bduffany]

Also, don't unnecessarily set attributes if we're not recording.

[Copilot]

Pull request overview

This PR expands OpenTelemetry root span enrichment using Bazel RequestMetadata, adding more queryable attributes to traces while avoiding attaching the full serialized metadata blob.

Changes:

• Extend request metadata extraction to also read RequestMetadata bytes from context.Context values (in addition to incoming gRPC metadata).
• Add TraceAttributes() and SetTraceAttributes() helpers to convert RequestMetadata into span attributes and apply them only when the span is recording.
• Refactor gRPC interceptors to use the new helper for propagating request metadata onto spans, and update Bazel deps accordingly.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
server/util/bazel_request/bazel_request.go	Adds trace attribute helpers and broadens where request metadata bytes can be sourced from (incoming metadata or context value).
server/util/bazel_request/BUILD	Adds OpenTelemetry attribute/trace deps required by the new helpers.
server/rpc/interceptors/interceptors.go	Switches span enrichment to call the new centralized request-metadata-to-span helper.

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