Metrics for for core distributor ops (#92)

This should give us a basic understanding of traffic over time, and allow us to compute basic error ratios.

cmd/internal/distributor/distributor.go

@@ -31,11 +31,43 @@ import (
 	"golang.org/x/mod/sumdb/note"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
 )
 
 // maxSigs is the maximum number of sigs that can be requested.
 const maxSigs = 100
 
+var (
+	counterCheckpointUpdateRequests = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "distributor_update_checkpoint_request",
+		Help: "The total number of requests to update a checkpoint",
+	})
+	counterCheckpointUpdateSuccess = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "distributor_update_checkpoint_success",
+		Help: "The total number of successful requests to update a checkpoint",
+	})
+
+	counterCheckpointGetNRequests = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "distributor_get_checkpoint_n_request",
+		Help: "The total number of requests to GetCheckpointN",
+	})
+	counterCheckpointGetNSuccess = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "distributor_get_checkpoint_n_success",
+		Help: "The total number of successful requests to GetCheckpointN",
+	})
+
+	counterCheckpointGetByWitRequests = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "distributor_get_checkpoint_wit_request",
+		Help: "The total number of requests to GetCheckpointWitness",
+	})
+	counterCheckpointGetByWitSuccess = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "distributor_get_checkpoint_wit_success",
+		Help: "The total number of successful requests to GetCheckpointWitness",
+	})
+)
+
 // NewDistributor returns a distributor that will accept checkpoints from
 // the given witnesses, for the given logs, and persist its state in the
 // database provided. Callers must call Init() on the returned distributor.
@@ -70,6 +102,7 @@ func (d *Distributor) GetLogs(ctx context.Context) ([]string, error) {
 
 // GetCheckpointN gets the largest checkpoint for a given log that has at least `n` signatures.
 func (d *Distributor) GetCheckpointN(ctx context.Context, logID string, n uint32) ([]byte, error) {
+	counterCheckpointGetNRequests.Inc()
 	if n == 0 || n > maxSigs {
 		return nil, status.Errorf(codes.InvalidArgument, "invalid N %d", n)
 	}
@@ -92,22 +125,30 @@ func (d *Distributor) GetCheckpointN(ctx context.Context, logID string, n uint32
 		}
 		return nil, fmt.Errorf("Scan(): %v", err)
 	}
+	counterCheckpointGetNSuccess.Inc()
 	return cp, nil
 }
 
 // GetCheckpointWitness gets the largest checkpoint for the log that was witnessed by the given witness.
 func (d *Distributor) GetCheckpointWitness(ctx context.Context, logID, witID string) ([]byte, error) {
+	counterCheckpointGetByWitRequests.Inc()
 	tx, err := d.db.BeginTx(ctx, &sql.TxOptions{ReadOnly: true})
 	if err != nil {
 		return nil, fmt.Errorf("failed to begin transaction: %v", err)
 	}
-	return getLatestCheckpoint(ctx, tx, logID, witID)
+	cp, err := getLatestCheckpoint(ctx, tx, logID, witID)
+	if err == nil {
+		counterCheckpointGetByWitSuccess.Inc()
+	}
+	return cp, err
 }
 
 // Distribute adds a new witnessed checkpoint to be distributed. This checkpoint must be signed
 // by both the log and the witness specified, and be larger than any previous checkpoint distributed
 // for this pair.
 func (d *Distributor) Distribute(ctx context.Context, logID, witID string, nextRaw []byte) error {
+	counterCheckpointUpdateRequests.Inc()
+
 	l, ok := d.ls[logID]
 	if !ok {
 		return fmt.Errorf("unknown log ID %q", logID)
@@ -154,6 +195,7 @@ func (d *Distributor) Distribute(ctx context.Context, logID, witID string, nextR
 				return fmt.Errorf("old checkpoint for tree size %d had hash %x but new one has %x", newCP.Size, oldCP.Hash, newCP.Hash)
 			}
 			// Nothing to do; checkpoint is equivalent to the old one so avoid DB writes.
+			counterCheckpointUpdateSuccess.Inc()
 			return nil
 		}
 	}
@@ -232,6 +274,7 @@ func (d *Distributor) Distribute(ctx context.Context, logID, witID string, nextR
 	if err := tx.Commit(); err != nil {
 		return err
 	}
+	counterCheckpointUpdateSuccess.Inc()
 	return nil
 }