Count various snapshot operations as they occur

#### Feature Description

It should be possible to build counters for snapshots operations e.g. `counter: consul.raft.snapshot_create` that have occurred during the agent's lifetime. We currently measure the time these operations take but do not count their frequency.

#### Use Case(s)

A counter of snapshot operations will give operators the opportunity to measure the rate that snapshots operations are occurring, which can help catch issues such as many small snapshots being created, or a repeated failed attempts to load snapshots.