Extend the auto-completion endpoint to provide auto-completion suggestions for metrics

[MariusBrill]

Currently the autocompleter does not autocomplete metrics. An autocompleter should be added which suggests key words for the metrics section.

[mariusoe]

Especially, for metric attributes in a rule block.

Example:

inspectit:
  instrumentation:
    rules:
      'r_test':
        metrics:
          'my_metric':
            ...
            <missing autocompletion>