Add autoupdate controller metrics

[hugoShaka]

Part of: RFD-184

Goal (internal): https://github.com/gravitational/cloud/issues/10289

This PR adds metrics to monitor and troubleshoot automatic agent rollouts. There are two metrics potentially increasing cardinality:

• the metrics containing labeled with the start or target versions. There a mechanism to cleanup old time series and remove older labels so they don't pile up.
• the metric containing the stage for each group. This metric is per-group, so as long as we control the number of groups we control the metric cardinality. The metric is the group name, so renaming groups will not increase cardinality and a misconfigured metrics server cannot accidentally disclose the agent groups.

Depends on:

• Use a non-global metrics registry in Teleport #50913
• Serve metrics from the local registry in the diagnostic service #51031