Allow any registered gRPC load balancer to be used (#8262)

gRPC-Go's `balancer` package includes a static
registration mechanism and a way to inspect whether a balancer name is
registered. We should use this mechanism instead of hard-coding an
allowlist of balancer names.

Custom collector configurations may have additional balancers linked in,
and we should allow them to be used.

.chloggen/any_balancer_name.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: configgrpc
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Allow any registered gRPC load balancer name to be used.
+
+# One or more tracking issues or pull requests related to the change
+issues: [8262]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

config/configgrpc/configgrpc.go

@@ -17,7 +17,7 @@ import (
 	"go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc"
 	"go.opentelemetry.io/otel"
 	"google.golang.org/grpc"
-	"google.golang.org/grpc/balancer/roundrobin"
+	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/credentials/insecure"
 	"google.golang.org/grpc/encoding/gzip"
@@ -38,9 +38,6 @@ import (
 
 var errMetadataNotFound = errors.New("no request metadata found")
 
-// Allowed balancer names to be set in grpclb_policy to discover the servers.
-var allowedBalancerNames = []string{roundrobin.Name, grpc.PickFirstBalancerName}
-
 // KeepaliveClientConfig exposes the keepalive.ClientParameters to be used by the exporter.
 // Refer to the original data-structure for the meaning of each parameter:
 // https://godoc.org/google.golang.org/grpc/keepalive#ClientParameters
@@ -269,12 +266,7 @@ func (gcs *GRPCClientSettings) toDialOptions(host component.Host, settings compo
 }
 
 func validateBalancerName(balancerName string) bool {
-	for _, item := range allowedBalancerNames {
-		if item == balancerName {
-			return true
-		}
-	}
-	return false
+	return balancer.Get(balancerName) != nil
 }
 
 // ToListener returns the net.Listener constructed from the settings.

config/configgrpc/configgrpc_test.go

@@ -18,6 +18,7 @@ import (
 	"go.uber.org/zap"
 	"go.uber.org/zap/zaptest/observer"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/peer"
 
@@ -35,6 +36,21 @@ import (
 	"go.opentelemetry.io/collector/pdata/ptrace/ptraceotlp"
 )
 
+// testBalancerBuilder facilitates testing validateBalancerName().
+type testBalancerBuilder struct{}
+
+func (testBalancerBuilder) Build(_ balancer.ClientConn, _ balancer.BuildOptions) balancer.Balancer {
+	return nil
+}
+
+func (testBalancerBuilder) Name() string {
+	return "configgrpc_balancer_test"
+}
+
+func init() {
+	balancer.Register(testBalancerBuilder{})
+}
+
 func TestDefaultGrpcClientSettings(t *testing.T) {
 	tt, err := obsreporttest.SetupTelemetry(component.NewID("component"))
 	require.NoError(t, err)
@@ -137,7 +153,7 @@ func TestAllGrpcClientSettings(t *testing.T) {
 				ReadBufferSize:  1024,
 				WriteBufferSize: 1024,
 				WaitForReady:    true,
-				BalancerName:    "round_robin",
+				BalancerName:    "configgrpc_balancer_test",
 				Authority:       "pseudo-authority",
 				Auth:            &configauth.Authentication{AuthenticatorID: component.NewID("testauth")},
 			},