[FR] Allow custom attributes on built-in traces (#6664)

[jrodiz]

[FR] Allow custom attributes on built-in traces (#6664)

Summary

Resolves #6664

FirebasePerformance already had putAttribute/removeAttribute/getAttribute/getAttributes backed by a ConcurrentHashMap, and TransportManager already attached those global attributes to every logged event — but the four methods were @hide, so developers could not actually set them.

This PR removes the @hide annotations, makes the related constants public, and adds the methods to the public API surface. No behavioral change is needed in the trace/network serialization path — TransportManager.setApplicationInfoAndBuild (firebase-perf/src/main/java/com/google/firebase/perf/transport/TransportManager.java:507-512) already calls putAllCustomAttributes(getGlobalCustomAttributes()) on ApplicationInfo for every trace metric and network request metric.

Where the attributes land

Global attributes attach at the ApplicationInfo.custom_attributes level, not at TraceMetric.custom_attributes / NetworkRequestMetric.custom_attributes. That distinction matters for any consumer reading the proto directly — those are separate proto fields. Trace- and request-level attributes set via Trace.putAttribute / HttpMetric.putAttribute continue to land in their respective per-event custom_attributes map and are independent of the global ones.

Usage

// In Application.onCreate() — set once, applies to all built-in and custom traces
// and to all network requests
FirebasePerformance.getInstance().putAttribute("build_type", "debug");
FirebasePerformance.getInstance().putAttribute("user_tier", "premium");

These attributes will appear on _app_start, _app_in_foreground, _app_in_background, screen traces (_st_*), network requests, and all custom traces automatically — via the existing TransportManager logic.

Notes

• Attribute limit: the global pool is validated at MAX_TRACE_CUSTOM_ATTRIBUTES (5) by FirebasePerformance.putAttribute's existing checkAttribute() call.
• Thread safety: mCustomAttributes is a ConcurrentHashMap; getAttributes() returns a defensive copy.
• Timing for _app_start: global attributes must be set before the trace is built (e.g., in Application.onCreate()).
• Backward compatible: all four methods already existed; only visibility was changed.

Verification

./gradlew :firebase-perf:test

All tests pass locally.

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[github-actions]

📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

[tejasd]

LGTM after - #8175