Add client_channel tracer; code cleanup; eliminate an allocation.

doc/environment_variables.md

@@ -42,6 +42,8 @@ some configuration as environment variables that can be set.
   - bdp_estimator - traces behavior of bdp estimation logic
   - call_error - traces the possible errors contributing to final call status
   - channel - traces operations on the C core channel stack
+  - client_channel - traces client channel activity, including resolver
+    and load balancing policy interaction
   - combiner - traces combiner lock state
   - compression - traces compression operations
   - connectivity_state - traces connectivity state changes to channels

src/core/ext/filters/client_channel/client_channel.h

@@ -23,6 +23,8 @@
 #include "src/core/ext/filters/client_channel/resolver.h"
 #include "src/core/lib/channel/channel_stack.h"
 
+extern grpc_tracer_flag grpc_client_channel_trace;
+
 // Channel arg key for server URI string.
 #define GRPC_ARG_SERVER_URI "grpc.server_uri"
 

src/core/ext/filters/client_channel/client_channel_plugin.c

@@ -78,6 +78,7 @@ void grpc_client_channel_init(void) {
       GRPC_CLIENT_CHANNEL, GRPC_CHANNEL_INIT_BUILTIN_PRIORITY, append_filter,
       (void *)&grpc_client_channel_filter);
   grpc_http_connect_register_handshaker_factory();
+  grpc_register_tracer("client_channel", &grpc_client_channel_trace);
 #ifndef NDEBUG
   grpc_register_tracer("resolver_refcount", &grpc_trace_resolver_refcount);
 #endif