Prometheus metrics support for continuous mode

[cstanislawski]

helpful for (and should be supported only in) the continuous mode: https://github.com/cstanislawski/grafana-db-exporter/issues/24

examples of metrics to export:

• grafana_dashboards_export_total - counter - total number of exports
• grafana_dashboards_export_errors_total - counter - total number of unsuccessful exports
• grafana_dashboard_last_export_timestamp - gauge - timestamp of the last dashboard export
• grafana_dashboard_last_successful_export_timestamp - gauge - timestamp of the last successful dashboard export
• grafana_dashboard_last_failed_export_timestamp - gauge - timestamp of the last unsuccessful dashboard export
• grafana_dashboards_commits_total - counter - number of dashboard changes
• grafana_dashboards_push_errors_total - counter - number of git push errors
• grafana_dashboards_api_requests_total - counter - total num of grafana api requests
• grafana_dashboards_api_request_errors_total - counter.- total number of unsuccessful grafana api requests
• grafana_dashboards_exporter_up - gauge - whether the exporter is up or not

not final metric names

[cstanislawski]

closing as #24 was closed

[cstanislawski]

nvm, continuous will be implemented