enhancement: Ability to set gRPC stats handler (#17)

Otel library has finally moved to using `stats.Handler` to record gRPC
traces and metrics. This means that we need a way for users to set a
`stats.Handler` on the connection established by the Cerbos client.

Signed-off-by: Charith Ellawala <[email protected]>

---------

Signed-off-by: Charith Ellawala <[email protected]>

cerbos/grpc.go

@@ -12,22 +12,25 @@ import (
 	"os"
 	"time"
 
-	effectv1 "github.com/cerbos/cerbos/api/genpb/cerbos/effect/v1"
-	enginev1 "github.com/cerbos/cerbos/api/genpb/cerbos/engine/v1"
-	requestv1 "github.com/cerbos/cerbos/api/genpb/cerbos/request/v1"
-	svcv1 "github.com/cerbos/cerbos/api/genpb/cerbos/svc/v1"
 	grpc_retry "github.com/grpc-ecosystem/go-grpc-middleware/v2/interceptors/retry"
 	"github.com/rs/xid"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/stats"
+
+	effectv1 "github.com/cerbos/cerbos/api/genpb/cerbos/effect/v1"
+	enginev1 "github.com/cerbos/cerbos/api/genpb/cerbos/engine/v1"
+	requestv1 "github.com/cerbos/cerbos/api/genpb/cerbos/request/v1"
+	svcv1 "github.com/cerbos/cerbos/api/genpb/cerbos/svc/v1"
 
 	"github.com/cerbos/cerbos-sdk-go/internal"
 )
 
 var _ Client[*GRPCClient, PrincipalCtx] = (*GRPCClient)(nil)
 
 type config struct {
+	statsHandler       stats.Handler
 	address            string
 	tlsAuthority       string
 	tlsCACert          string
@@ -133,6 +136,13 @@ func WithUnaryInterceptors(interceptors ...grpc.UnaryClientInterceptor) Opt {
 	}
 }
 
+// WithStatsHandler sets the gRPC stats handler for the connection.
+func WithStatsHandler(handler stats.Handler) Opt {
+	return func(c *config) {
+		c.statsHandler = handler
+	}
+}
+
 // New creates a new Cerbos client.
 func New(address string, opts ...Opt) (*GRPCClient, error) {
 	grpcConn, _, err := mkConn(address, opts...)
@@ -172,6 +182,10 @@ func mkConn(address string, opts ...Opt) (*grpc.ClientConn, *config, error) {
 func mkDialOpts(conf *config) ([]grpc.DialOption, error) {
 	dialOpts := []grpc.DialOption{grpc.WithUserAgent(conf.userAgent)}
 
+	if conf.statsHandler != nil {
+		dialOpts = append(dialOpts, grpc.WithStatsHandler(conf.statsHandler))
+	}
+
 	if conf.connectTimeout > 0 {
 		dialOpts = append(dialOpts, grpc.WithConnectParams(grpc.ConnectParams{MinConnectTimeout: conf.connectTimeout}))
 	}