SAMZA-2706: Clean up specific handling of diagnostics-specific metrics reporter

[cameronlee314]

Issues: Specific checks for the diagnosticsreporter metrics reporter were making the code for setting up diagnostics and the diagnosticsreporter metrics reporter a bit harder to work with. For the new StaticResourceJobCoordinator, it made object wiring harder for diagnostics-related components. #1021 initially had a reason to have special logic for creating the diagnosticsreporter metrics reporter, but #1369 made that special logic unnecessary.

Changes:

1. Changed DiagnosticsUtil.buildDiagnosticsManager to only return a DiagnosticsManager instead of Pair<DiagnosticsManager, MetricsSnapshotReporter>.
2. MetricsReporterLoader.getMetricsReporters no longer excludes the diagnosticsreporter metrics reporter. It just creates all configured reporters.
3. Since MetricsReporterLoader.getMetricsReporters returns all reporters, any classes that use that method no longer need to inject the diagnosticsreporter explicitly.

Tests:
./gradlew build

API changes:

1. In the YARN application master,diagnosticsreporter metrics reporter now is given a processorId of "ApplicationMaster" instead of "samza-container-ApplicationMaster".
2. When using MetricsReporterLoader.getMetricsReporters, if diagnosticsreporter is in metrics.reporters, then diagnosticsreporter metrics reporter is always created.
3. The diagnosticsreporter metrics reporter may still be created even if job.diagnostics.enabled is set to false. Note that Samza will only automatically create the stream for the diagnosticsreporter metrics reporter if diagnostics is enabled (since the DiagnosticsManager also uses that same stream), so if there is a case in which diagnosticsreporter metrics reporter is needed while diagnostics is disabled, then the stream for the reporter needs to be created through some other means.

[rmatharu-zz]

LGTM,

I'll create a jira for writing an integ test that tests out the behavior of data being emitted into diag-stream.
Currently we rely on rel-test for this.