grpc: client/server histogram bucket change (#7591)

Changes the default grpc client/server histogram buckets from the
defaults to better track the long tail of slow requests. Removes `.005`
and `.25` granularity in favor of adding the larger values of `45` and `90`
to avoid changing the cardinality.

```
# Before, the default prometheus buckets
[]float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10}

# After
[]float64{.01, .025, .05, .1, .5, 1, 2.5, 5, 10, 45, 90}
```

Fixes https://github.com/letsencrypt/boulder/issues/6384

grpc/client.go

@@ -85,7 +85,9 @@ type clientMetrics struct {
 func newClientMetrics(stats prometheus.Registerer) (clientMetrics, error) {
 	// Create the grpc prometheus client metrics instance and register it
 	grpcMetrics := grpc_prometheus.NewClientMetrics(
-		grpc_prometheus.WithClientHandlingTimeHistogram(),
+		grpc_prometheus.WithClientHandlingTimeHistogram(
+			grpc_prometheus.WithHistogramBuckets([]float64{.01, .025, .05, .1, .5, 1, 2.5, 5, 10, 45, 90}),
+		),
 	)
 	err := stats.Register(grpcMetrics)
 	if err != nil {

grpc/server.go

@@ -292,7 +292,9 @@ type serverMetrics struct {
 func newServerMetrics(stats prometheus.Registerer) (serverMetrics, error) {
 	// Create the grpc prometheus server metrics instance and register it
 	grpcMetrics := grpc_prometheus.NewServerMetrics(
-		grpc_prometheus.WithServerHandlingTimeHistogram(),
+		grpc_prometheus.WithServerHandlingTimeHistogram(
+			grpc_prometheus.WithHistogramBuckets([]float64{.01, .025, .05, .1, .5, 1, 2.5, 5, 10, 45, 90}),
+		),
 	)
 	err := stats.Register(grpcMetrics)
 	if err != nil {