HTTP/2 flow control tracing

subchannel_flowctrl tracer, if enabled, logs local and remote window
sizes of subchannel's HTTP2 session to debug log on the start of every
call.

doc/environment_variables.md

@@ -37,6 +37,7 @@ can be set.
   - `server_call` - Traces server handling of individual requests
   - `subchannel` - Traces subchannel connectivity state and errors
   - `subchannel_refcount` - Traces subchannel refcount changes
+  - `subchannel_flowctrl` - Traces HTTP/2 flow control
 
   The following tracers are added by the `@grpc/grpc-js-xds` library:
   - `cds_balancer` - Traces the CDS load balancing policy

packages/grpc-js/src/subchannel.ts

@@ -41,6 +41,7 @@ import { SubchannelRef, ChannelzTrace, ChannelzChildrenTracker, SubchannelInfo,
 const clientVersion = require('../../package.json').version;
 
 const TRACER_NAME = 'subchannel';
+const FLOW_CONTROL_TRACER_NAME = 'subchannel_flowctrl';
 
 const MIN_CONNECT_TIMEOUT_MS = 20000;
 const INITIAL_BACKOFF_MS = 1000;
@@ -324,6 +325,10 @@ export class Subchannel {
     logging.trace(LogVerbosity.DEBUG, 'subchannel_refcount', '(' + this.channelzRef.id + ') ' + this.subchannelAddressString + ' ' + text);
   }
 
+  private flowControlTrace(text: string): void {
+    logging.trace(LogVerbosity.DEBUG, FLOW_CONTROL_TRACER_NAME, '(' + this.channelzRef.id + ') ' + this.subchannelAddressString + ' ' + text);
+  }
+
   private handleBackoffTimer() {
     if (this.continueConnecting) {
       this.transitionToState(
@@ -848,6 +853,12 @@ export class Subchannel {
         ' with headers\n' +
         headersString
     );
+    this.flowControlTrace(
+      'local window size: ' +
+        this.session!.state.localWindowSize +
+        ' remote window size: ' +
+        this.session!.state.remoteWindowSize
+    );
     const streamSession = this.session;
     let statsTracker: SubchannelCallStatsTracker;
     if (this.channelzEnabled) {