feat(perf): add support for a custom HttpOverrides implementation to automatically trace all Http requests in Dart

[Salakar]

Description

This PR is a proof of concept way of adding support for automatic http traces for Firebase Perf in Dart.

Usage

import 'dart:io';
import 'package:flutter/material.dart';

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_performance/firebase_performance.dart';
import 'package:firebase_performance/http_overrides.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  HttpOverrides.global = FirebasePerformanceHttpOverrides(
    FirebasePerformance.instance,
    // (optional) An example of adding custom attributes based on request data
    onRequest: (HttpClientRequest request, HttpMetric metric) {
      metric.putAttribute('custom_attribute', 'custom_value');
    },
    // (optional) An example of adding custom attributes based on response data
    onResponse: (HttpClientResponse response, HttpMetric metric) {
      response.headers.value('foo') ??
          metric.putAttribute('bar', response.headers.value('bar')!);
    },
  );
  runApp(MyApp());
}

API

/// A function that is called when a HTTP request is created.
/// This is useful for adding custom metric attributes to the
/// provided [HttpMetric], for example;
/// ```dart
/// final onRequest = (HttpClientRequest request, HttpMetric metric) {
///   metric.putAttribute('custom_attribute', 'custom_value');
/// }
/// ```
typedef HttpMetricOnRequestInterceptor = void Function(
  HttpClientRequest request,
  HttpMetric metric,
);

/// A function that is called when a HTTP response is received.
/// This is useful for adding custom metric attributes to the
/// provided [HttpMetric], for example;
/// ```dart
/// final onResponse = (HttpClientResponse response, HttpMetric metric) {
///   metric.putAttribute('custom_attribute', 'custom_value');
/// }
/// ```
typedef HttpMetricOnResponseInterceptor = void Function(
  HttpClientResponse response,
  HttpMetric metric,
);

/// Implementation of Darts [HttpOverrides] that allows for automatic capturing of
/// HTTP requests and responses via Firebase Performance [HttpMetric]'s.
class FirebasePerformanceHttpOverrides extends HttpOverrides {
  // ...
}

Related Issues

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Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.