[PMM-13141] Expose featureCompatibilityVersion

[tumd]

Is your feature request related to a problem? Please describe.

1. Before upgrading to a new major version of a node/replica you must verify that all nodes have correct featureCompatibilityVersion set on all nodes.
   It would be convenient to only have to glance a Grafana dashboard to assert this is correctly set on all nodes before upgrade.
2. It would be nice to setup an alert that triggers if "featureCompatibilityVersion" and mongodb_version_info{mongodb="x.y.z"} differ, notifying you that you have forgotten to update this after a major version upgrade.

Describe the solution you'd like
Either add a new metric which expose a label with the value of featureCompatibilityVersion, or add this label to the already existing mongodb_version_info metric.

db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

Describe alternatives you've considered
None at the moment.

Additional context
None at the moment.