Document removal of _total from metric name.

It's only in the timeseries name now.

Author: brian-brazil

README.md

@@ -72,6 +72,9 @@ c.inc()     # Increment by 1
 c.inc(1.6)  # Increment by given value
 ```
 
+If there is a suffix of `_total` on the metric name, it will be removed. This
+is for compatibility between OpenMetrics and the Prometheus text format.
+
 There are utilities to count exceptions raised:
 
 ```python