[FR] Allow custom attributes on built-in traces via global attribute API

[jrodiz]

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

Summary

Resolves #6664

Firebase Performance already had putAttribute/getAttribute/removeAttribute/getAttributes on FirebasePerformance backed by a ConcurrentHashMap, but these were @hide and not wired into built-in trace serialization. This change makes them public and ensures global attributes are included on all traces and network requests — built-in and custom alike.

Changes

• FirebasePerformance.java — Made 4 global attribute methods (putAttribute, removeAttribute, getAttribute, getAttributes) and 3 constants (MAX_TRACE_CUSTOM_ATTRIBUTES, MAX_ATTRIBUTE_KEY_LENGTH, MAX_ATTRIBUTE_VALUE_LENGTH) public
• api.txt — Added the 4 methods to the public FirebasePerformance API surface
• TraceMetricBuilder.java — Merges global attributes into all Trace-based metrics (screen traces, custom traces); trace-level attributes override global on key conflict
• AppStartTrace.java — Applies global attributes to the _app_start trace metric
• AppStateMonitor.java — Applies global attributes to foreground (_fs) and background (_bs) session traces
• NetworkRequestMetricBuilder.java — Merges global attributes into all network request metrics (auto-instrumented OkHttp and manual HttpMetric); per-request attributes override global on key conflict

Usage

// In Application.onCreate() — set once, applies to all built-in and custom traces
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.

Design Notes

• Attribute precedence: trace/request-level attributes override global attributes with the same key
• Attribute limit: global attributes are validated at 5 max by existing checkAttribute() logic
• Thread safety: mCustomAttributes is a ConcurrentHashMap; getAttributes() returns a defensive copy
• Timing for _app_start: global attributes must be set before the first activity resumes (e.g., in Application.onCreate())
• Backward compatible: all methods already existed; only visibility was changed

Test Coverage

• _app_start — AppStartTraceTest
• _app_in_foreground (_fs) — AppStateMonitorTest
• _app_in_background (_bs) — AppStateMonitorTest
• Screen traces (_st_*) — AppStateMonitorTest
• Network requests — NetworkRequestMetricBuilderTest
• All events via ApplicationInfo — TransportManagerTest

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