Tolerate running applications when initialising metrics tests

The entire metrics_c2s_SUITE used to be skipped, since
metrics_helper:start_lhttpc attempts to start five applications, exiting
with a badmatch if any of them are already running, which crypto,
public_key and ssl are.  This patch makes it accept 'already_started'
results from application:start.

Once we drop support for anything older than R16B01, we could use
application:ensure_started/1 here instead.