Add cmdline args to support provision for flow/hlm.

Summary:
Adds args '--enable-flow-metrics' and '--enable-server-metrics', which can be used together or separately.  See Boundary-3598.

Opted to not use 'getopt' as it's not clear to me if it's installed on all systems setup_meter.sh might run on.

Test Plan: Make sure it works as advertised.

Reviewers: busterb, thirty8super

Differential Revision: http://phabricator.cid1.boundary.com/D2863