Use _total suffix for counters

According to the project's naming best practices[0], the suffix to
indicate a counter type is called `_total`.

[0]: https://prometheus.io/docs/practices/naming/

README.md

@@ -130,7 +130,7 @@ wildcard match in the matching line. That allows for dynamic rewrites, such as:
 ```yaml
 mappings:
 - match: test.*.*.counter
-  name: "${2}_counter"
+  name: "${2}_total"
   labels:
     provider: "$1"
 ```
@@ -142,7 +142,7 @@ could be written as:
 mappings:
 - match: test\.(\w+)\.(\w+)\.counter
   match_type: regex
-  name: "${2}_counter"
+  name: "${2}_total"
   labels:
     provider: "$1"
 ```
@@ -152,6 +152,7 @@ label names.
 
 If the default metric help text is insufficient for your needs you may use the YAML
 configuration to specify a custom help text for each mapping:
+
 ```yaml
 mappings:
 - match: http.request.*