Make all registry methods thread safe

The registry is backed by a Hash, which is not guaranteed to be thread
safe on all interpreters. For piece of mind, this change synchronizes
all accesses to the metrics hash.

Another option would have been to use a [thread-safe Hash][1] instead of
a Hash but this would have meant adding Ruby Concurrent as a
dependency, which I'm assuming we don't want.

Ref: #184 (comment)

[1]: https://github.com/ruby-concurrency/concurrent-ruby/blob/v1.1.7/lib/concurrent-ruby/concurrent/hash.rb

Signed-off-by: Matthieu Prat <matthieuprat@gocardless.com>

lib/prometheus/client/registry.rb

@@ -22,7 +22,7 @@ def register(metric)
         name = metric.name
 
         @mutex.synchronize do
-          if exist?(name.to_sym)
+          if @metrics.key?(name.to_sym)
             raise AlreadyRegisteredError, "#{name} has already been registered"
           end
           @metrics[name.to_sym] = metric
@@ -73,15 +73,15 @@ def histogram(name, docstring:, labels: [], preset_labels: {},
       end
 
       def exist?(name)
-        @metrics.key?(name)
+        @mutex.synchronize { @metrics.key?(name) }
       end
 
       def get(name)
-        @metrics[name.to_sym]
+        @mutex.synchronize { @metrics[name.to_sym] }
       end
 
       def metrics
-        @metrics.values
+        @mutex.synchronize { @metrics.values }
       end
     end
   end