common: Allow PerfCounters to return a provided service ID

common: Allow PerfCounters to return a provided service ID

Dashboard has asked for a unique identifier that can be associated
with services. This commit provides a component of that
functionality. Enforcing uniqueness is beyond the scope of this PR and
is the responsibility of cluster setup and orchestration. The scope of
uniqueness is a matter of policy and up to the design of cluster setup
and orchestration software.

We provide the `--service_unique_id` argument that can be passed on
the command line when executing a Ceph service that uses
`global_init`. If non-empty, a `service_unique_id` section is added to
the PerfCounters dump for that service. This section has a single
entry whose name is set to the argument of `service_unique_id` and
whose value is arbitrary. If unspecified or empty, no
`service_unique_id` section is added.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>