Fix race condition

[DarthMax]

There is a race condition in the following functions

• Tabs#increment_counter
• Tabs#record_value
• Tabs#start_task

All function check if the requested metric exist. If not they try to create it, but in the mean time it is possible that an other thread has already created it. This results in the following code to fail repeatedly with a Tabs::DuplicateMetricError

5.times.map do 
  Thread.new do
    Tabs.increment_counter("test_counter")
  end
end.each(&:join)

Adding a get_or_create_metric function using a shared mutex prevents this race condition.

[DarthMax]

I have noticed, that this might not be the best attempt to solve the issue, as it will not prevent the race condition it multiple distinct programs try to access the same metrics. Furthermore will the the mutex slow down all metric operations, although the race condition is only present when the metric is first accessed.

A better approach might be to prevent the increment_counter, record_value and start_task functions from trying to create a not existing metric.