Add support for monitoring arbitrary blocks of code using performance monitoring

To be able to more accurately measure performance of blocks of code we'll need to have a way to monitor whatever happens in a Ruby block. For example:

Gitlab::Metrics.measure_block('the_name_of_the_series') do
  ...
end

One use case of this: being able to measure the amount of calls to the /allowed API calls. Right now we can't do this as InfluxDB doesn't allow searching by unindexed values and Grape only provides URLs (not action names) which aren't indexed.

cc @pcarranza

Also consider adding just a counter to see how many times a block is executed. Maybe this is included by default.

mentioned in merge request !3515 (merged)

Status changed to closed by merge request !3515 (merged)

mentioned in commit 755edd80