feat(job-processor): report attempts metrics (#448)

## What ❔

Every component implementing JobProcessor now exposes two metrics:

`job_attempts` -- heatmaps in grafana will be added displaying this
metric.
`max_attempts_reached` -- alert will fire when this metric is increased
(alert rules not added yet).

Also, `attempts` columns in DB were increased in both `get_next*`
methods and `requeue_stuck_jobs`, so it was basically `attempts += 2` on
retry. I removed `attempts` update in `requeue_stuck_jobs`.

## Why ❔

Better monitoring and alerting

## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog
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core/bin/contract-verifier/src/verifier.rs

@@ -524,4 +524,12 @@ impl JobProcessor for ContractVerifier {
         // Do nothing
         Ok(())
     }
+
+    fn max_attempts(&self) -> u32 {
+        u32::MAX
+    }
+
+    async fn get_job_attempts(&self, _job_id: &Self::JobId) -> anyhow::Result<u32> {
+        Ok(1)
+    }
 }