raft: Add metrics monitoring performance of core Raft processing

The following metrics are designed to monitor the correctness,
consistency, and performance of Gitaly. The context is that a storage
can host hundreds of thousands of partitions.

* gitaly_raft_proposal_duration_seconds
Type: Histogram (exponential buckets)
Labels: storage
Description: Time to commit proposals (sampled)
Purpose: Track write latency distribution

* gitaly_raft_proposals_total
Type: Counter
Labels: storage, result (success/error)
Description: Counter of all Raft proposals
Purpose: Track overall throughput and error rates

* gitaly_raft_log_entries_processed
Type: Counter
Labels: storage, operation (append/commit/apply), entry_type
Description: Rate of log entries processed
Purpose: Monitor overall system throughput