Tracer registration.

First: ugh.

Second: allow tracer global variables to be registered and then parsed out of a configuration environment variable.

Expose TSI trace config variable directly to ease this a little.
diff --git a/src/core/channel/channel_stack.h b/src/core/channel/channel_stack.h
index c136f5c..ef1da7b 100644
--- a/src/core/channel/channel_stack.h
+++ b/src/core/channel/channel_stack.h
@@ -298,7 +298,9 @@
 void grpc_call_element_send_cancel(grpc_call_element *cur_elem);
 void grpc_call_element_send_finish(grpc_call_element *cur_elem);
 
+extern int grpc_trace_channel;
+
 #define GRPC_CALL_LOG_OP(sev, elem, op) \
-  if (grpc_trace_bits & GRPC_TRACE_CHANNEL) grpc_call_log_op(sev, elem, op)
+  if (grpc_trace_channel) grpc_call_log_op(sev, elem, op)
 
 #endif  /* GRPC_INTERNAL_CORE_CHANNEL_CHANNEL_STACK_H */