Include number of maximum active streams in exception message (#11644)

Motivation:

When users receive "Maximum active streams violated for this endpoint"
exception, it's useful to know what is the current max streams limit on
HTTP/2 connection.

Modifications:

- Include current number of maximum active streams in exception message;

Result:

Easier debugging of HTTP/2 connections.

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2Connection.java

@@ -474,8 +474,10 @@ public final <V> V removeProperty(PropertyKey key) {
         @Override
         public Http2Stream open(boolean halfClosed) throws Http2Exception {
             state = activeState(id, state, isLocal(), halfClosed);
-            if (!createdBy().canOpenStream()) {
-                throw connectionError(PROTOCOL_ERROR, "Maximum active streams violated for this endpoint.");
+            final DefaultEndpoint<? extends Http2FlowController> endpoint = createdBy();
+            if (!endpoint.canOpenStream()) {
+                throw connectionError(PROTOCOL_ERROR, "Maximum active streams violated for this endpoint: " +
+                        endpoint.maxActiveStreams());
             }
 
             activate();
@@ -888,7 +890,8 @@ private void checkNewStreamAllowed(int streamId, State state) throws Http2Except
             }
             boolean isReserved = state == RESERVED_LOCAL || state == RESERVED_REMOTE;
             if (!isReserved && !canOpenStream() || isReserved && numStreams >= maxStreams) {
-                throw streamError(streamId, REFUSED_STREAM, "Maximum active streams violated for this endpoint.");
+                throw streamError(streamId, REFUSED_STREAM, "Maximum active streams violated for this endpoint: " +
+                        (isReserved ? maxStreams : maxActiveStreams));
             }
             if (isClosed()) {
                 throw connectionError(INTERNAL_ERROR, "Attempted to create stream id %d after connection was closed",