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>

Author: matthieuprat

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