pkg/log: ability for debug logs (#1689)

* pkg/log: ability for debug logs

When the debug plugin is enabled all log.Debug calls will print to
standard; if not there are a noop (almost).

The log package wraps some standard log functions as well, so just
replacing "log" with "plugin/pkg/log" should be enough to use this
package.

* docs

* Add docs

* lint

* Test fallthrough to log pkg as well

* simple package - up test coverage

* add other log levels as well

* update docs

1 files changed, 8 insertions, 4 deletions

diff --git a/plugin.md b/plugin.md
index 3e0bb86e9..e94de6d1c 100644
--- a/plugin.md
+++ b/plugin.md
@@ -35,11 +35,15 @@ See a couple of blog posts on how to write and add plugin to CoreDNS:
 
 ## Logging
 
-If your plugin needs to output a log line you should use the `log` package. CoreDNS does not
-implement log levels. The standard way of outputing is: `log.Printf("[LEVEL] ...")`, and LEVEL
-can be: `INFO`, `WARNING` or `ERROR`.
+If your plugin needs to output a log line you should use the `plugin/pkg/log` package. This package
+implements log levels. The standard way of outputting is: `log.Info` for info level messages. The
+levels available are `log.Info`, `log.Warning`, `log.Error`, `log.Debug`. Each of these also has
+a `f` variant.
+
 In general, logging should be left to the higher layers by returning an error. However, if there is
-a reason to consume the error but notify the user, then logging in the plugin can be acceptable.
+a reason to consume the error and notify the user, then logging in the plugin itself can be
+acceptable. The `Debug*` functions only output something when the *debug* plugin is loaded in the
+server.
 
 ## Metrics