PMM-4154 Add log-levels. (#143)

Author: Anton Kucherov

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Added Authentification Database option when connect to mongo #139, thx [@etiennecoutaud](https://github.com/etiennecoutaud).
 - Added helm chart to readme #140, thx [@pgdagenais](https://github.com/pgdagenais).
+- [PMM-4154](https://jira.percona.com/browse/PMM-4154): Added standard logging flags.
 
 ### Fixed
 - Fixed some function comments based on best practices from Effective Go #137, thx [@CodeLingoBot](https://github.com/CodeLingoBot).

mongodb_exporter.go

@@ -78,6 +78,7 @@ var (
 
 func main() {
 	initVersionInfo()
+	log.AddFlags(kingpin.CommandLine)
 	kingpin.Parse()
 
 	if *testF {

testdata/mongodb_exporter.testFlagHelp.golden

@@ -56,4 +56,11 @@ Flags:
                                    Valid time units are 'ns', 'us' (or 'µs'), 'ms', 's', 'm', 'h'.
       --groups.enabled=""        Currently ignored
       --version                  Show application version.
+      --log.level="info"         Only log messages with the given severity or
+                                 above. Valid levels: [debug, info, warn, error,
+                                 fatal]
+      --log.format="logger:stderr"  
+                                 Set the log target and format. Example:
+                                 "logger:syslog?appname=bob&local=7" or
+                                 "logger:stdout?json=true"