enhancement(unit tests): Add testing for component specification features (#9312)

* Add support for a thread-local internal metrics recorder

* Move metrics controller functionality into `struct Controller`

* Rename `metrics::init` to `init_global` to clarify usage

* Convert all tests to use the thread-local metrics controller

* Add support for recording and testing emission of actual events

Signed-off-by: Bruce Guenter <bruce.guenter@datadoghq.com>