Avoid rolling up an ERROR state when empty GenericAnalyzer blocks are marked discard_stale, or when all of their items are STALE.

[asymingt]

Add logic to a parent AnalyzerGroup to prevent rolling up an ERROR state when a GenericAnalyzer child block is marked with discard_stale: true and either of these conditions are met:

1. There are no statuses that match any of the GenericAnalyzer child blocks (its size is zero).
2. Every matching status in the GenericAnalyzer child block has been marked stale.

In this example, if bar and baz have no matching statuses or all of their statuses are STALE, they will roll up as OK because the discard_stale: true flag implies that stale statuses are disposable.

diagnostics_aggregator:
  ros__parameters:
    pub_rate: 1.0
    path: 'robot'
    analyzers:
      part:
        type: 'diagnostic_aggregator/AnalyzerGroup'
        path: 'part'
        foo:
          type: 'diagnostic_aggregator/GenericAnalyzer'
          path: 'foo'
          find_and_remove_prefix: ['/foo:']
          num_items: 1
        bar:
          type: 'diagnostic_aggregator/GenericAnalyzer'
          path: 'bar'
          find_and_remove_prefix: ['/bar:']
          discard_stale: true
        baz:
          type: 'diagnostic_aggregator/GenericAnalyzer'
          path: 'baz'
          find_and_remove_prefix: ['/baz:']
          discard_stale: true

[ct2034]

@asymingt - thanks a lot for your contribution!
Please also add a test case that checks for this and that defines the desired behavior.

[asymingt]

@asymingt - thanks a lot for your contribution! Please also add a test case that checks for this and that defines the desired behavior.

I have done my best to add one, given the limited time available to me!

[ct2034]

@asymingt I think I am happy with this now. But could you please have look at the comments and changes I made in 6e852dc and just confirm if they make sende to you?

[asymingt]

@asymingt I think I am happy with this now. But could you please have look at the comments and changes I made in 6e852dc and just confirm if they make sende to you?

Yup, those changes make sense. LGTM. Thanks, @ct2034.