RUMM-319 Improve logging for app extensions

[ncreated]

What and why?

🚚 This PR addresses the enhancement discussed in #52 - sending logs immediately.

The origin of this enhancement idea was the use case of running the SDK in short-lived app extension. Default timing configuration for persistence and upload was not effective when running in such environment so the SDK did not always have enough time to upload logs to Datadog during app extension lifetime.

However, instead of implementing "send logs immediately" feature we decided to adjust timing configuration if SDK is running in app extension.

How?

An Environment abstraction is introduced:

/// An environment running the SDK.
internal enum Environment: Equatable {
    /// iOS app
    case app
    /// iOS app extension
    case appExtension
}

The environment is determined automatically during the Datadog.initialize() call and provides custom timing configuration for each case. The .app environment defaults to previous configuration and .appExtension brings distinct values.

The .appExtension environment:

• Issues the initial upload right away: 0.5s after the SDK is initialized. This enables even very-short lived extensions to upload logs collected but not-yet-uploaded in the previous session.
• Defaults to 5s for max upload delay. This means, every check will be performed in less than 5s (vs 20s in .app environment).
• Delay decrease factor is more aggressive: 0.5 (vs 0.9 in .app). This significantly reduces the time for next upload if the extension performs intensive logging.

A note on integration tests

💡 This PR adds only unit tests and necessary changes to benchmarks. It doesn't ship the integration test for app extension. Doing this will require changing the nature of our integration tests - from being a unit test target to UI Test target which launches the app and app extension. Not done in this PR, as I don't want to conflict with other open PRs.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference

[buranmert]

The concerns of Envrionment and Strategy types seem to be kind of mixed up to me.
I'd like to propose another way for this change:

// no logic inside, simply an indicator of which environment we are in
enum Environment {
  case app
  case extension
}

struct DefaultLogsUploadStrategy: LogsUploadStrategy {
  let internal = default_interval // normal/default interval, eg: for app
  ...
}

struct IntensiveLogsUploadStrategy: LogsUploadStrategy {
  let internal = very_short_interval // a very short interval, eg: for app ext
  ...
}
// this could be LogsUploadStrategyBuilder or just an internal function instead of an extension
extension LogsUploadStrategy {
  static func build(for environment: Envrionment) -> LogsUploadStrategy {
    if app { return DefaultLogsUploadStrategy() }
    else { return IntensiveLogsUploadStrategy() }
  }
}

IMO that keeps concerns separate and strategies extensible (ie: currently, what happens if we'd like intensive loggin in apps?)

what do you think?

[ncreated]

Closed in favour of #84