fix: correct metrics label in requestPreVote handler (hashicorp#665)

The requestPreVote handler reuses the metrics key "requestVote" from copy-paste in hashicorp#530. This causes pre-vote RPC latency to be mixed into the raft.rpc.requestVote metric.

Fix: change to "requestPreVote" to match the pattern used by all other RPC handlers.

Co-authored-by: Qian-Cheng-nju <Qian-Cheng-nju@users.noreply.github.com>

Author: Qian-Cheng-nju

raft.go

@@ -1735,7 +1735,7 @@ func (r *Raft) requestVote(rpc RPC, req *RequestVoteRequest) {
 
 // requestPreVote is invoked when we get a request Pre-Vote RPC call.
 func (r *Raft) requestPreVote(rpc RPC, req *RequestPreVoteRequest) {
-	defer metrics.MeasureSince([]string{"raft", "rpc", "requestVote"}, time.Now())
+	defer metrics.MeasureSince([]string{"raft", "rpc", "requestPreVote"}, time.Now())
 	r.observe(*req)
 
 	// Setup a response