Add performance_schema.status_by_host gauge

[romuald]

This commit aims at adding a gauge measuring the contents of the
performance_schema.status_by_host table

Gauge name is mysql_perf_schema_status_by_host,
with labels host and variable_name

[romuald]

Please note that this is my first PR on some Go code in general. I've simply copied one of the other modules, trying to rename things properly.
I'm also unaware of some possible naming conventions used in the project

Tested locally with a Percona server 5.7

[romuald]

poke @SuperQ because I forgot to read the CONTRIBUTING before opening the PR

[SuperQ]

Can you describe more about what this table is useful for in a monitoring context?

[romuald]

@SuperQ in our case, we had a some SQL clients doing "bad" queries and increasing the read_rnd_next handlers (mysql_global_status_handlers_total{handler="read_rnd_next"})

Monitoring this table helped us to narrow the host / process behaving poorly

Did you mean to add this context in the commit or files too?