Document histogram consistency limitations (#161)

README.md

@@ -348,6 +348,10 @@ Records a value in a previously registered histogram. Usually called from
 [log_by_lua_block](https://github.com/openresty/lua-nginx-module#log_by_lua_block)
 globally or per server/location.
 
+Note that recording an observation requires incrementing several histogram,
+which does not happen atomically and might race with metric collection (see
+#161).
+
 * `value` is a value that should be recorded. Required.
 * `label_values` is an array of label values.