[llvm] [CI] Add queue size, running count metrics (PR #122714)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

Tested locally by setting the 3 env variables, new metrics are showing in Grafana
```
$ export GRAFANA_METRICS_USERID=XXX
$ export GRAFANA_API_KEY=XXX
$ export GITHUB_TOKEN=XXX
$ python3 metrics.py
Uploaded 7 metrics
```






https://github.com/llvm/llvm-project/pull/122714