Update database metric prefix

Avoid conflict with postgres_exporter `pg_`.

README.md

@@ -18,9 +18,9 @@ metrics.
 
 1. Database
     * [Per-table tuple stats](lib/gitlab_monitor/database/tuple_stats.rb) --
-      `pg_stat_table_*`
+      `gitlab_database_stat_table_*`
     * [Row count queries](lib/gitlab_monitor/database/row_count.rb) --
-      `db_rows_count`
+      `gitlab_database_rows`
     * [CI builds](lib/gitlab_monitor/database/ci_builds.rb) --
       `ci_pending_builds`, `ci_created_builds`, `ci_stale_builds`,
       `ci_running_builds`

lib/gitlab_monitor/database/tuple_stats.rb

@@ -35,7 +35,7 @@ module GitLab
 
           result.each do |table_name, tuple_stats|
             tuple_stats.each do |column_name, value|
-              @metrics.add("pg_stat_table_#{column_name}", value.to_s, table_name: table_name)
+              @metrics.add("gitlab_database_stat_table_#{column_name}", value.to_s, table_name: table_name)
             end
           end