Metrics persist even when queries are no longer active

[LazarenkoA]

Hi there,
First of all, I apologize for posting this as an issue, but since there's no discussions section in the repository, I thought I'd bring it up here.

Issue: I'm using postgres_exporter with a custom queries.yaml file to track slow queries in my PostgreSQL database. The problem I'm encountering is that once a query appears on the dashboard, it remains there indefinitely, even after it's no longer active or being executed. The issue persists despite using GAUGE for the mean_time metric, which I expected would allow for dynamic changes and removal of inactive queries from the dashboard.

Here’s a simplified version of my custom query in queries.yaml:

pg_slow_queries:
  query: "SELECT case when length(query) > 200 then concat(left(query, 100), ' + ... + ', right(query, 100), ' - ' ,md5(query)) else query end as query, max(total_time) as total_time, max(mean_time) as mean_time, max(calls) as calls FROM pg_stat_statements WHERE total_time > 500 group by case when length(query) > 200 then concat(left(query, 100), ' + ... + ', right(query, 100), ' - ' ,md5(query)) else query end;"
  metrics:
    - query:
        usage: "LABEL"
        description: "Query"
    - total_time:
        usage: "COUNTER"
        description: "Total time for slow queries"
    - calls:
        usage: "COUNTER"
        description: ""
    - mean_time:
        usage: "GAUGE"
        description: "Mean time for slow queries"

Even after slow queries are no longer running, they still appear on the dashboard. I would expect that with GAUGE, these metrics would disappear or reset if the corresponding query is no longer present in pg_stat_statements.

Additional question: I noticed that the --extend.query-path flag is marked as deprecated. Could you provide some guidance on the recommended approach for extending queries now that this flag is deprecated? How should we properly use custom queries going forward?