xds: Make timestamp tracking variables volatile in ExternalProcessorClientInterceptor (grpc#13003)

Make timestamp tracking variables volatile in
ExternalProcessorClientInterceptor to fix data race reported by TSAN.
These fields track timestamps for recording latency metrics. They were
being read and written concurrently across multiple threads:

The application thread calling the gRPC stub (e.g. start and halfClose).
The data plane response thread (e.g., `onHeaders` and `onClose`).
The external processor client response thread processing the response
callback (e.g. `onNext` which invokes `proceedWithHalfClose`,
`proceedWithHeaders`, etc.).

Author: kannanjgithub

xds/src/main/java/io/grpc/xds/ExternalProcessorClientInterceptor.java

@@ -297,10 +297,10 @@ private static class DataPlaneClientCall
     private final String backendService;
     private volatile Context callContext = Context.ROOT;
 
-    private long clientHeadersStartNanos;
-    private long clientHalfCloseStartNanos;
-    private long serverHeadersStartNanos;
-    private long serverTrailersStartNanos;
+    private volatile long clientHeadersStartNanos;
+    private volatile long clientHalfCloseStartNanos;
+    private volatile long serverHeadersStartNanos;
+    private volatile long serverTrailersStartNanos;
 
     private boolean protocolConfigSent = false;
     private ImmutableMap<String, Struct> collectedAttributes;