lua: bump metrics module

Bump metrics package submodule. There are 8 new commits, but only
two of them affects Tarantool:
- Add election_leader_idle metric [1];
- test: run log capture tests on a separate server [2];
the other 6 affect documentation and CI/CD scripts of the original repo.
The latter one is required to bump luatest in test-run [3], since
otherwise metrics log capture test fails on a new version.

1. tarantool/metrics@ba9726d
2. tarantool/metrics@3370f85
3. tarantool/test-run#426

NO_DOC=doc is a part of submodule
NO_TEST=repo runs submodule tests, no new test files are added

(cherry picked from commit 50f7425)

changelogs/unreleased/bump-metrics-to-3370f85.md

@@ -0,0 +1,3 @@
+## feature/lua
+
+* Added a `tnt_election_leader_idle` metric to built-in `metrics`.