Adds note about status cause

instrumentation/grpc/src/main/java/brave/grpc/GrpcParser.java

@@ -14,6 +14,7 @@
 package brave.grpc;
 
 import brave.ErrorParser;
+import brave.Span;
 import brave.SpanCustomizer;
 import brave.Tracing;
 import brave.internal.Nullable;
@@ -50,19 +51,18 @@ protected <ReqT, RespT> String spanName(MethodDescriptor<ReqT, RespT> methodDesc
 
   /**
    * Override to change what data from the status or trailers are parsed into the span modeling it.
-   * By default, this tags "grpc.status_code" when it is not OK.
+   * By default, this tags "grpc.status_code" when it is not OK, and "error" if there was no {@link
+   * Status#getCause()}.
+   *
+   * <p><em>Note</em>: {@link Status#getCause()} will be set as {@link Span#error(Throwable)} by
+   * default. You don't need to parse it here.
    */
   protected void onClose(Status status, Metadata trailers, SpanCustomizer span) {
     if (status == null || status.isOk()) return;
 
     String code = String.valueOf(status.getCode());
     span.tag("grpc.status_code", code);
-    Throwable cause = status.getCause();
-    if (cause != null) {
-      errorParser().error(status.getCause(), span);
-    } else {
-      span.tag("error", code);
-    }
+    if (status.getCause() == null) span.tag("error", code);
   }
 
   static @Nullable String method(String fullMethodName) {