(Feature)|Surface request metrics to response middleware

[eneko]

• I've read, understood, and done my best to follow the CONTRIBUTING guidelines.

This pull request includes (pick all that apply):

• Bugfixes
• New features
• Breaking changes
• Documentation updates
• Unit tests
• Other

Summary

In order to track request duration and other metrics, we want to expose this information via response middleware.

Requests metrics are provided by URLSessionDataTask via URLSessionDataDelegate (see changes in URLSessionClient.swift, line 319):

/// Stores request metrics in task response, for later consumption.
/// This delegate method is always called after `didReceive response`, and before `didCompleteWithError`.
@available(iOS 10.0, *)
func urlSession(_ session: URLSession, task: URLSessionTask, didFinishCollecting metrics: URLSessionTaskMetrics) {
    var taskResponse = taskResponseFor(taskIdentifier: task.taskIdentifier)
    taskResponse.metrics = metrics
    update(taskResponse: taskResponse, for: task.taskIdentifier)
}

In the delegate method above, we store the collected metrics inside the task response structure. This structure is then passed to all response middleware in synchronouslyProcessResponseMiddleware().

Breaking changes

• Applications and/or frameworks implementing ResponsePipelineMiddleware will have to be updated.

Implementation

• SessionDelegate has been updated to capture request metrics in TaskResponse.
• URLSessionClient has been updated to pass TaskResponse to any response middleware.
• ResponsePipelineMiddleware has been refactored to pass a TaskResponse structure, which contains response data, HTTP response, error, and metrics, where available.
• Existing unit tests have been updated accordingly, and new tests have been added.
• Private URL extension has been moved to test targets.
• @testable imports have been removed where possible.

Test Plan

• Run all tests

[tiltem]

Very cool, URLSessionTaskMetrics FTW! Nice work

[tiltem]

👍

[johnhammerlund]

Great work! Just one comment regarding accessibility modifiers.

R~

[johnhammerlund]

These should be public, minus the expectedContentLength

[eneko]

Oh, good catch! Saved me from doing a patch release :)