Address trace echo review feedback

Summary: Follow up on D112132643 by moving shared observer dependencies into UCACHE_BASE_DEPS, passing loggingEnabled through the low-level context constructor, correcting naming and declaration documentation, and clarifying why malformed trace context is caught at the request boundary.

Reviewed By: alikhtarov

Differential Revision: D113599571

fbshipit-source-id: 90e0b79071bcd5193bdc8100597bbead058d6dfc

Author: John Hugg

mcrouter/lib/network/FBTrace-inl.h

@@ -61,7 +61,7 @@ std::shared_ptr<TracingData> traceRequestReceived(
 // path (no cost counters, no scribe flush). Stores the tracer under the
 // response-context key so getCurrentTracer()/replyImpl echoes it back. Returns
 // true if a tracer was set up.
-// NOTE: this function does not exist if LIBMC_FBTRACE_DISABLE is defined.
+// When LIBMC_FBTRACE_DISABLE is defined, the inline stub above returns false.
 bool traceEchoOnlyRequestReceived(const std::string& traceContext);
 
 #endif