Add ws_thread_overflow counter

See details in r02275.vtc for why there isn't a test case for this.

bin/varnishd/VSC_main.vsc

@@ -608,6 +608,13 @@
 
 	Number of times we ran out of space in workspace_client.
 
+.. varnish_vsc:: ws_thread_overflow
+	:level: diag
+	:group: wrk
+	:oneliner: workspace_thread overflows
+
+	Number of times we ran out of space in workspace_thread.
+
 .. varnish_vsc:: shm_records
 	:level:	diag
 	:oneliner:	SHM records

bin/varnishd/cache/cache_wrk.c

@@ -394,6 +394,8 @@ Pool_Work_Thread(struct pool *pp, struct worker *wrk)
 			tp = &tpx;
 		} while (tp->func != NULL);
 
+		if (WS_Overflowed(wrk->aws))
+			wrk->stats->ws_thread_overflow++;
 		/* cleanup for next task */
 		wrk->seen_methods = 0;
 	}