feat(client): Add support for monitoring gRPC endpoints (#1376)

* add grpc

* add gRPC to readme

client/grpc.go

@@ -0,0 +1,71 @@
+package client
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+	"time"
+
+	"github.com/TwiN/logr"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/credentials/insecure"
+	health "google.golang.org/grpc/health/grpc_health_v1"
+)
+
+// PerformGRPCHealthCheck dials a gRPC target and performs the standard Health/Check RPC.
+// Returns whether a connection was established, the serving status string, an error (if any), and the elapsed duration.
+func PerformGRPCHealthCheck(address string, useTLS bool, cfg *Config) (bool, string, error, time.Duration) {
+	if cfg == nil {
+		cfg = GetDefaultConfig()
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), cfg.Timeout)
+	defer cancel()
+
+	var opts []grpc.DialOption
+	// Transport credentials
+	if useTLS {
+		tlsCfg := &tls.Config{InsecureSkipVerify: cfg.Insecure}
+		if cfg.HasTLSConfig() && cfg.TLS.isValid() == nil {
+			tlsCfg = configureTLS(tlsCfg, *cfg.TLS)
+		}
+		opts = append(opts, grpc.WithTransportCredentials(credentials.NewTLS(tlsCfg)))
+	} else {
+		opts = append(opts, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	}
+	// Custom dialer for DNS resolver or SSH tunnel
+	opts = append(opts, grpc.WithContextDialer(func(ctx context.Context, addr string) (net.Conn, error) {
+		if cfg.ResolvedTunnel != nil {
+			return cfg.ResolvedTunnel.Dial("tcp", addr)
+		}
+		if cfg.HasCustomDNSResolver() {
+			resolverCfg, err := cfg.parseDNSResolver()
+			if err != nil {
+				// Shouldn't happen because already validated; log and fall back
+				logr.Errorf("[client.PerformGRPCHealthCheck] invalid DNS resolver: %v", err)
+			} else {
+				d := &net.Dialer{Resolver: &net.Resolver{PreferGo: true, Dial: func(ctx context.Context, network, _ string) (net.Conn, error) {
+					d := net.Dialer{}
+					return d.DialContext(ctx, resolverCfg.Protocol, resolverCfg.Host+":"+resolverCfg.Port)
+				}}}
+				return d.DialContext(ctx, "tcp", addr)
+			}
+		}
+		var d net.Dialer
+		return d.DialContext(ctx, "tcp", addr)
+	}))
+
+	start := time.Now()
+	conn, err := grpc.DialContext(ctx, address, opts...)
+	if err != nil {
+		return false, "", err, time.Since(start)
+	}
+	defer conn.Close()
+
+	client := health.NewHealthClient(conn)
+	resp, err := client.Check(ctx, &health.HealthCheckRequest{Service: ""})
+	if err != nil {
+		return false, "", err, time.Since(start)
+	}
+	return true, resp.GetStatus().String(), nil, time.Since(start)
+}