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