add system metadata labels

[cczhuang420]

This PR is to export the system metadata labels along with metric and resource labels.

[SuperQ]

Sorry, this is a job or service discovery.

System metadata labels are defined by Prometheus, not by exporters.

[SuperQ]

Sorry, I misunderstood this change. I think this needs more documentation and description as to why it is necessary.

[avivzgroundcover]

@SuperQ We also found it missing when using the project. System and user labels are part of the resources being monitored, and there's many use cases requiring filtering / grouping metrics by those types of labels.

@cczhuang420 Could I ask that you also add user defined labels in this PR? As described in the API description, MonitoredResourceMetadata includes both systemLabels which you added but also userLabels.
https://cloud.google.com/monitoring/api/ref_v3/rest/v3/TimeSeries#MonitoredResourceMetadata

[cczhuang420]

@SuperQ We also found it missing when using the project. System and user labels are part of the resources being monitored, and there's many use cases requiring filtering / grouping metrics by those types of labels.

@cczhuang420 Could I ask that you also add user defined labels in this PR? As described in the API description, MonitoredResourceMetadata includes both systemLabels which you added but also userLabels. https://cloud.google.com/monitoring/api/ref_v3/rest/v3/TimeSeries#MonitoredResourceMetadata

Hi folks, sorry that I couldn't manage to finish this given my current limited bandwidth, so have moved it to draft. I would need to test it more throughly before raising a PR. Feel free to close it.