enhancement(observability): More debug info on more HTTP requests (#4999)

* Add more debug info to all requests sent by HttpClient

* Use HttpClient in prometheus exporter integration test

* Use HttpClient in GCP token fetch

* Give HttpClient::send a concrete error type

* Move HttpClient::call into an internal non-mut function

This makes a lot of uses of HttpClient non-mut as well.

* Use HttpError for all error returns

* Use HttpClient in prometheus source

* Use HttpClient in apache metrics source

* Format the body by showing the size instead of just debug

Signed-off-by: Bruce Guenter <bruce@timber.io>