[llvm] [CI] Always send a heartbeat metric (PR #122708)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

Tested locally by setting the 3 env variables:
```
$ export GRAFANA_METRICS_USERID=XXX
$ export GRAFANA_API_KEY=XXX
$ export GITHUB_TOKEN=XXX
$ python3 metrics.py
Uploaded 3 metrics
```

https://github.com/llvm/llvm-project/pull/122708