unblocking eval doesn't decrease `blocked_evals_total_blocked` metric

### Nomad version
`Nomad v1.4.1 (2aa7e66bdb526e25f59883952d74dad7ea9a014e)`

### Operating system and Environment details
RHEL 7.9
Docker 20.10.14

### Issue
The following metric appears to be a counter, as it shows an evaluation blocked - we definitely had one blocked, but have since resolved it.
```
# HELP nomad_nomad_blocked_evals_total_blocked nomad_nomad_blocked_evals_total_blocked
# TYPE nomad_nomad_blocked_evals_total_blocked gauge
nomad_nomad_blocked_evals_total_blocked{host="somehost"} 1
```

```
larsonb►nomad eval list -status Pending
No evals found
larsonb►nomad eval list -status Blocked
No evals found
```

### Reproduction steps
Block an eval and see if it returns to 0 or increments.
#### Expected Result
Should either be a gauge or a counter

#### Actual Result
Seems to act like a counter.

Start by reproducing the blocked-evaluation lifecycle and inspecting where the blocked_evals_total_blocked metric is registered and updated in Nomad. Confirm whether it is intended to be a gauge or counter, then verify the metric after an evaluation is blocked and subsequently unblocked; done means its behavior and HELP/TYPE output match the chosen semantics.

Written by the indexing model from the issue text.