Changes

s1gw_metrics: more precise type hints

This enables dialyzer to detect errors when:

* ctr_* API is used on an ?S1GW_GAUGE_* definition, and
* gauge_* API is used on an ?S1GW_CTR_* definition.

One of such errors has been fixed in a previous commit.

Change-Id: I36eaa5e04037e165e43eb960c541f967c394a2ea