Add ability to reset metrics between crawl to limit payload absorbed (#5)

by the prometheus/victoria metric crawlers.

README.md

@@ -23,6 +23,7 @@ This project is in active use by [telegram-bot.app](https://telegram-bot.app), a
   - [API endpoints](#api-endpoints)
     - [Ban or unban the user](#ban-or-unban-the-user)
   - [General](#general)
+    - [Metrics which matter](#metrics-which-matter)
     - [Healthcheck](#healthcheck)
     - [Monitoring endpoint](#monitoring-endpoint)
 
@@ -123,6 +124,7 @@ In this case, both proxy and websockets will be available under the `/v1/graphql
 | `API_PORT`                | The port to expose the monitoring API   | `9090`                     |
 | `BANNED_USERS_FILE`       | The path to the file with banned users  | `/go/src/app/banned_users.json`   |
 | `PROXIED_CLIENT_TIMEOUT` | The timeout for the proxied client in seconds     | `120`                      |
+| `PURGE_METRICS_ON_CRAWL` | Purge metrics on each /metrics crawl    | `false`                      |
 
 ### Speed
 
@@ -227,6 +229,12 @@ Ban details will be stored in the `banned_users.json` file, which you can mount
 
 ### General
 
+#### Metrics which matter
+
+You can always enable `PURGE_METRICS_ON_CRAWL` environment variable to purge the metrics on each `/metrics` crawl. This will allow you to see only the current metrics, without potential leftovers from the previous crawls. This is useful if you want to monitor the metrics in real-time and / or limit the amount of data ingested into the monitoring system. When enabled you will most likely need to update your monitoring queries.
+
+With the `PURGE_METRICS_ON_CRAWL` enabled, the `graphql_proxy_requests_failed`, `graphql_proxy_requests_skipped` and `graphql_proxy_requests_succesful` metrics will remain between resets.
+
 #### Healthcheck
 
 If you'd like the `/healthz` endpoint to perform actual check for the connectivity to the graphql endpoint - set the `HEALTHCHECK_GRAPHQL_URL` environment variable to the exact URL of the graphql endpoint. The query executed will be `query { __typename }` and if the response is not `200 OK` - the healthcheck will fail. Remember that the endpoint is a full URL which you'd like to check, so it should include the protocol, host and path - for example `http://localhost:8080/v1/graphql` and it's NOT the same as value of `HOST_GRAPHQL` environment variable which should provide only the host, without path, ending with slash.