+ Use NullMetrics to mock metrics when unused
+ Use method_missing in NullMetrics mocking
+ Update prometheus gem to version that correctly uses transitive dependencies
+ Ensure correct folders are used in Multiprocess prometheus client tests.
+ rename Sessions controller's metric

This is a step for #29118.

Add a single metric to count successful logins.

Summary types are not supported so remove Collector.  Either
we need to support the summary type or we need to create a
multiprocess-friendly Collector.

Add config to load prometheus and set up the Collector and the
Exporter.

Fix `Gemfile` as current prometheus-client gemspec is missing the
`mmap2` dependency.

This reverts merge request !11645

Signed-off-by: NRémy Coutable <remy@rymai.me>

Closes gitaly#199

It was only available for test and development environments and we need
it to run the `rake gettext:pack` command in omnibus which doesn't have
the test/dev gems installed.

This should fix the extra backslashes in e-mail names as well
as other things: https://github.com/mikel/mail/blob/master/CHANGELOG.rdoc#head

See https://github.com/mperham/sidekiq/blob/master/Changes.md#4210

This reverts merge request !10589

Fixes LoadError after local compilation.

https://github.com/grpc/grpc/issues/9998

This gem prevents Mash from responding to :permitted?, disabling mass
assignment protection for the Grape API

Closes gitaly#173