contrib/google.golang.org/grpc: use service name from function (#613)

This commit changes the service name in a call to startSpanFromContex from
cfg.serviceName to cfg.serverServiceName(). Everywhere else in the grpc
server integration, the service name is taken from cfg.serverServiceName(),
except this one place which erroneously uses cfg.serviceName directly.

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

@@ -83,7 +83,7 @@ func StreamServerInterceptor(opts ...Option) grpc.StreamServerInterceptor {
 				ctx,
 				info.FullMethod,
 				"grpc.server",
-				cfg.serviceName,
+				cfg.serverServiceName(),
 				tracer.AnalyticsRate(cfg.analyticsRate),
 				tracer.Measured(),
 			)