contrib/google.golang.org/grpc.v12: update grpc contrib to work with …

…metadata API changes.

Google's metadata API has changed, and so our grpc.v12 contrib no longer compiles.
This commit updates the grpc.v12 contrib package to use the new API.

contrib/google.golang.org/grpc.v12/grpc.go

@@ -51,7 +51,7 @@ func startSpanFromContext(ctx context.Context, method, service string, rate floa
 	if !math.IsNaN(rate) {
 		opts = append(opts, tracer.Tag(ext.EventSampleRate, rate))
 	}
-	md, _ := metadata.FromContext(ctx) // nil is ok
+	md, _ := metadata.FromIncomingContext(ctx) // nil is ok
 	if sctx, err := tracer.Extract(grpcutil.MDCarrier(md)); err == nil {
 		opts = append(opts, tracer.ChildOf(sctx))
 	}
@@ -81,12 +81,12 @@ func UnaryClientInterceptor(opts ...InterceptorOption) grpc.UnaryClientIntercept
 			spanopts = append(spanopts, tracer.Tag(ext.EventSampleRate, cfg.analyticsRate))
 		}
 		span, ctx = tracer.StartSpanFromContext(ctx, "grpc.client", spanopts...)
-		md, ok := metadata.FromContext(ctx)
+		md, ok := metadata.FromOutgoingContext(ctx)
 		if !ok {
 			md = metadata.MD{}
 		}
 		_ = tracer.Inject(span.Context(), grpcutil.MDCarrier(md))
-		ctx = metadata.NewContext(ctx, md)
+		ctx = metadata.NewOutgoingContext(ctx, md)
 		opts = append(opts, grpc.Peer(&p))
 		err := invoker(ctx, method, req, reply, cc, opts...)
 		if p.Addr != nil {

internal/version/version.go

@@ -8,4 +8,4 @@ package version
 // Tag specifies the current release tag. It needs to be manually
 // updated. A test checks that the value of Tag never points to a
 // git tag that is older than HEAD.
-const Tag = "v1.20.0"
+const Tag = "v1.21.0"