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`.

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