To avoid typos and for a little extra type-safety.

Not used by the API any more.

The profiler sets agentURL to http://host:port/profiling/v1/input.
So doing path.Join on that to add the telemetry proxy endpoint is
wrong for two reasons: The "//" at the beginning will be converted
to a "/", and the endpoint will be wrong. We need to parse the
URL and replace the path with the telemetry proxy endpoint.

In particular, explain what the default http.Client will be and why to
use a non-default http.Client

…metry

…metry

The debug feature (which causes the backend to emit more verbose
logging) is only intended for development. There's no point having it be
a public member of the telemetry Client since it can be enabled via an
environment variable for development.