Add metrics about changes of deployment (#169)

* Add the repo_id field to DeploymentStatistics

* Add the 'production_environment' field

* Add the 'rollback' field to count rollback

* Fix to produce statistics only for the production environment

* Produce statistics of changes of deployment from the latest deployment

* Add docs of metrics

* Add the  'IsProductionEnvironment' method to fix test

Author: Noah Hanjun Lee

docs/concepts/metrics.md

@@ -28,10 +28,14 @@ scrape_configs:
 
 ## Gitploy Metrics
 
-Gitploy provides the following Gitploy metrics:
+Gitploy provides the following Gitploy metrics. *Note that Some metrics are provided only for the production environment* (i.e. `production_environment: true` in the configuration file).
 
 * **gitploy_requests_total** <br/> How many HTTP requests processed, partitioned by status code and HTTP method.
 * **gitploy_request_duration_seconds**<br/> The HTTP request latencies in seconds.
-* **gitploy_deployment_count**<br/> The total count of success deployment for each environment, respectively.
+* **gitploy_deployment_count** <br/> The count of success deployment of the production environment.
+* **gitploy_rollback_count**<br/> The count of rollback of the production environment.
+* **gitploy_line_additions**<br/> The count of added lines from the latest deployment of the production environment.
+* **gitploy_line_deletions**<br/> The count of deleted lines from the latest deployment of the production environment.
+* **gitploy_line_changes**<br/> The count of changed lines from the latest deployment of the production environment.
 * **gitploy_member_count**<br/> The total count of members.
-* **gitploy_member_limit**<br/> The limit count of members.
+* **gitploy_member_limit**<br/> The limit count of members.