Use monotonic clock when timestamping observed values

The Monotonic clock is going to be more accurate on the few cases where
the distinction matters, but it's also somehow faster than `Time.now`.

Signed-off-by: Daniel Magliola <danielmagliola@gocardless.com>

lib/prometheus/client/data_stores/direct_file_store.rb

@@ -279,9 +279,10 @@ def write_value(key, value)
  init_value(key)
  end
 
+ now = Process.clock_gettime(Process::CLOCK_MONOTONIC)
  pos = @positions[key]
  @f.seek(pos)
- @f.write([value, Time.now.to_f].pack('dd'))
+ @f.write([value, now].pack('dd'))
  @f.flush
  end