Add concurrent-ruby gem

The reason we need this gem is to be able to provide a good concurrency
story in our "Data Stores".

We want the data stores to be thread-safe to make it easy for metrics to
use them, and to shield away the complexity so that the store can do what's
most efficient, and metrics don't need to make assumptions on how it
works.

However, we also need metrics to be able to update multiple values at once
so in some cases (most notably, histograms), the store does need to provide
a synchronization method.

Since the normal `set` / `increment` methods call a Mutex already, having
a Mutex block around them means we end up calling `Mutex.synchronize` twice,
which *should* work, but Ruby Mutexes are not reentrant.

`concurrent-ruby` provides a Reentrant lock. It's a Read-Write Lock, not
a Mutex, but by always grabbing Write locks, it's functionally the same

Signed-off-by: Daniel Magliola <[email protected]>

prometheus-client.gemspec

@@ -14,4 +14,6 @@ Gem::Specification.new do |s|
 
   s.files             = %w(README.md) + Dir.glob('{lib/**/*}')
   s.require_paths     = ['lib']
+
+  s.add_dependency 'concurrent-ruby'
 end