[VAULT-17826] Remove mount point from rollback metrics

[miagilepner]

This PR creates two new metrics, vault.route.rollback and vault.rollback.attempt. These metrics replace the existing vault.route.rollback.{MOUNT_POINT} and vault.rollback.attempt.{MOUNT_POINT} metrics. By default, the metrics with the mount point in them are disabled, however they can be enabled with:

telemetry {
  add_mount_point_rollback_metrics = true
}

[github-actions]

Build Results:
All builds succeeded! ✅

[github-actions]

CI Results:
All Go tests succeeded! ✅

[mpalmi]

Looks great! Nice docs and test improvements.

nit: Should we include some warning about the cardinality implications of enabling the mount-specific metrics in the docs?

[schavis]

Doc changes LGTM. Feel free to merge once the ENG review is complete