Add doc for binary size measurement. (#8930)

* Add doc for binary size measurement.

Author: granluo

scripts/health_metrics/README.md

@@ -1,12 +1,15 @@
-# Code Coverage Report Generation
+# SDK Health Metrics Report Generation
+This directory includes code coverage report generation and binary size report generation.
+
+## Code Coverage Report Generation
 
 This tool is to help generate coverage reports for pull requests. It's defined by the [test_coverage  workflow](https://github.com/firebase/firebase-ios-sdk/blob/master/scripts/health_metrics/code_coverage_file_list.json).
 
 Coverage reports of SDK frameworks will be displayed in a pull request if the change is under corresponding SDK file patterns.
 
 [UpdatedFilesCollector](https://github.com/firebase/firebase-ios-sdk/tree/master/scripts/health_metrics/generate_code_coverage_report/Sources/UpdatedFilesCollector) will detect file changes and compare file paths to file patterns in [code_coverage_file_list.json](https://github.com/firebase/firebase-ios-sdk/blob/master/scripts/health_metrics/code_coverage_file_list.json). If updated file paths fit any patterns, corresponding SDK coverage job will be triggered.
 
-## Add a new coverage workflow
+### Add a new coverage workflow
 
 To create a code coverage workflow for a new SDK,
 1. Add `newsdk` and its patterns in [code_coverage_file_list.json](https://github.com/firebase/firebase-ios-sdk/blob/master/scripts/health_metrics/code_coverage_file_list.json).
@@ -35,3 +38,35 @@ pod-lib-lint-newsdk:
 Add the job name to the [`needs` of `create_report` job](https://github.com/firebase/firebase-ios-sdk/blob/64d50a7f7b3af104a88f9c9203285ae20ea309d4/.github/workflows/test_coverage.yml#L277).
 
 4. If this newsdk podspec has unit test setup, e.g. [database](https://github.com/firebase/firebase-ios-sdk/blob/64d50a7f7b3af104a88f9c9203285ae20ea309d4/FirebaseDatabase.podspec#L44-L57), with `unit_tests.scheme = { :code_coverage => true }`, the code coverage workflow should run unit tests through podspecs and utilize those coverage data and nothing is needed to update here. Otherwise, add [another way of running tests](https://github.com/firebase/firebase-ios-sdk/blob/64d50a7f7b3af104a88f9c9203285ae20ea309d4/scripts/health_metrics/pod_test_code_coverage_report.sh#L26) and generating xcresult bundles with code coverage data in pod_test_code_coverage_report.sh.
+
+## Binary Size Report Generation
+
+This tool is to generate a report for SDK binary sizes.
+[create_binary_size_report.sh](https://github.com/firebase/firebase-ios-sdk/blob/master/scripts/health_metrics/create_binary_size_report.sh)
+will trigger [BinarySizeReportGenerator](https://github.com/firebase/firebase-ios-sdk/blob/master/scripts/health_metrics/generate_code_coverage_report/Sources/BinarySizeReportGenerator/BinarySizeReportGeneration.swift)
+which will run [cocoapods-size](https://github.com/google/cocoapods-size)
+to create a binary size report and either post it in PRs in presubmits or
+upload it to the Metrics Service.
+
+### Replicate a binary size measurement for a released SDK
+
+To replicate an SDK binary size measurement locally, we can run the following
+script to get the binary size of the latest released FirebaseABTesting SDK.
+```
+git clone https://github.com/google/cocoapods-size
+cd cocoapods-size
+python3 measure_cocoapod_size.py --cocoapods FirebaseABTesting
+```
+
+More about CocoaPods Size measurement could be found [here](https://github.com/google/cocoapods-size#cocoapods-size-measurement)
+
+### Environment Setup
+
+The `binary_size_metrics` on GitHub Actions [workflow](https://github.com/firebase/firebase-ios-sdk/blob/master/.github/workflows/health-metrics-presubmit.yml)
+is running binary size measurement presubmits and postsubmits. The environment for this job
+could be found in `runs-on`. More details about the envionment could be found [here](https://github.com/actions/virtual-environments#available-environments).
+
+Factors below might also affect the binary size:
+- Each testapp is measured by a default version Xcode from the GitHub Actions environment above.
+- Architecture is set to `arm64` and the destination is `generic/platform=iOS Simulator` when a testapp is built.
+- The source of dependencies is the [SpecStaging repo](https://github.com/firebase/SpecsStaging), which is for prerelease, and [CocoaPods CDN](https://cdn.cocoapods.org).