Add metrics for requests forwarded by standbys. (hashicorp#11366)

changelog/11366.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+core: Add metrics for standby node forwarding.
+```

vault/request_forwarding.go

@@ -14,9 +14,9 @@ import (
 	"sync"
 	"time"
 
+	"github.com/armon/go-metrics"
 	log "github.com/hashicorp/go-hclog"
 	"github.com/hashicorp/vault/helper/forwarding"
-
 	"github.com/hashicorp/vault/sdk/helper/consts"
 	"github.com/hashicorp/vault/vault/cluster"
 	"github.com/hashicorp/vault/vault/replication"
@@ -336,6 +336,8 @@ func (c *Core) ForwardRequest(req *http.Request) (int, http.Header, []byte, erro
 		return 0, nil, nil, ErrCannotForward
 	}
 
+	defer metrics.MeasureSince([]string{"ha", "rpc", "client", "forward"}, time.Now())
+
 	origPath := req.URL.Path
 	defer func() {
 		req.URL.Path = origPath
@@ -354,6 +356,7 @@ func (c *Core) ForwardRequest(req *http.Request) (int, http.Header, []byte, erro
 	}
 	resp, err := c.rpcForwardingClient.ForwardRequest(req.Context(), freq)
 	if err != nil {
+		metrics.IncrCounter([]string{"ha", "rpc", "client", "forward", "errors"}, 1)
 		c.logger.Error("error during forwarded RPC request", "error", err)
 		return 0, nil, nil, fmt.Errorf("error during forwarding RPC request")
 	}