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

Replace this paragraph with a list of issues related to this PR from the issue database. Indicate, which of these issues are resolved or fixed by this PR. Note that you'll have to prefix the issue numbers with flutter/flutter#.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• [ ] 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.

[russellwheatley]

This is a POC.