Provide useful cluster names for non k8s environments on managed prom…

[jsuereth]

…etheus exporter.

• we use __run__ on cloud run and __gce__ on compute engine.
• This matches the documentation for cloud run and support in Ops Agent.
• This can be controlled via a configuration parameter.

cc @ridwanmsharif

Note: This just modifies the defaults, happy to provide docs or transition period.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a227a23) 69.85% compared to head (1dc879f) 69.92%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #776 +/- ## ========================================== + Coverage 69.85% 69.92% +0.06% ========================================== Files 42 42 Lines 4863 4874 +11 ========================================== + Hits 3397 3408 +11 Misses 1315 1315 Partials 151 151 ```

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