Make census_tag_set the only field of census_context

src/core/census/context.h

@@ -39,11 +39,7 @@
 /* census_context is the in-memory representation of information needed to
  * maintain tracing, RPC statistics and resource usage information. */
 struct census_context {
-  gpr_uint64 op_id;    /* Operation identifier - unique per-context */
-  gpr_uint64 trace_id; /* Globally unique trace identifier */
-                       /* TODO(aveitch) Add census tags:
-                          const census_tag_set *tags;
-                        */
+  census_tag_set *tags;  /* Opaque data structure for census tags. */
 };
 
 #endif /* GRPC_INTERNAL_CORE_CENSUS_CONTEXT_H */