1.0: metrics: Add more metrics local behavior descriptions

Signed-off-by: Hiroshi Hatake <[email protected]>

Author: cosmo0920

metrics/README.md

@@ -32,6 +32,11 @@ Here is an example with `metrics_local`:
 </system>
 ```
 
+`local` type plugin should provide equivalent behavior before Fluentd v1.13.
+This metrics type should provide single numeric value storing functionality.
+
+And this `local` type plugin should be used by default.
+
 ## List of Built-in Metrics Plugins
 
 * [`local`](local.md)

metrics/local.md

@@ -26,4 +26,7 @@ With this configuration:
 
 The above configuration will save the internal metrics for plugins on memory. As a result, you can retrive metrics from memory and also you can replace with your custom metrics plugin.
 
+Actually, @type local metrics plugin has equivalent functionality for previous single value based Ruby instance variables.
+This behavior will be changed by other 3rd party plugins.
+
 If this article is incorrect or outdated, or omits critical information, please [let us know](https://github.com/fluent/fluentd-docs-gitbook/issues?state=open). [Fluentd](http://www.fluentd.org/) is an open-source project under [Cloud Native Computing Foundation \(CNCF\)](https://cncf.io/). All components are available under the Apache 2 License.