feat: rename TraceAgent/TraceApi to Tracer

[kjin]

BREAKING CHANGE: TraceAgent has been renamed to Tracer. In plugins, Patch has been renamed Monkeypatch, and Patch is now Monkeypatch|Intercept (this is a rename of Instrumentation). There are no user-visible JS changes.

TraceAgent is a misnomer since the module as a whole is the "agent", not the object that is returned when the Trace Agent is started. (This is already reflected in the docs, where it has been called TraceApi). This change makes things consistent so it is called Tracer everywhere.

The one implementation of TraceAgent (which was also called TraceAgent) is now StackdriverTracer. This object is never constructed by the user, so this is a no-op change for JS users.

A couple of unused imports have also been removed.

[ofrobots]

LGTM if you modify the commit message to make it clearly in the the first sentence that the breaking change only is limited to the exported types and no JS-exposed names and/or functionality is changing.