ethdb/pebble: fix pebble metrics registration

[magicxyyz]

Pebble gauge metrics are stuck when opening a database for the second time (within the same metrics namespace, after closing first one).

Gauge metrics are created with metrics.NewRegisteredGauge and it fails silently if metric name already exists. The Meter metrics work despite that metrics.NewRegisteredMeter is used, because it calls metrics.GetOrRegisterMeter internally.

This PR fixes the issue by using metrics.GetOrRegister* functions instead.

[rjl493456442]

Sorry, what's the issue fixed by this pull request? As you said, GetOrRegisterMeter call GetOrRegister internally and these two should be equivalent, isn't it?

[magicxyyz]

Sorry, what's the issue fixed by this pull request? As you said, GetOrRegisterMeter call GetOrRegister internally and these two should be equivalent, isn't it?

metrics.NewRegisteredMeter and metrics.GetOrRegisterMeter are equivalent, but metrics.NewRegisteredGauge and metrics.GetOrRegisterGauge are not - the former fails silently if metric name already exists and that causes the issue mentioned in this PR's description:

Pebble gauge metrics are stuck when opening a database for the second time (within the same metrics namespace, after closing first one).