Add mechanism to map a processes env variable to a Prometheus metric

[nfisher]

Given a PyTorch process with PID 1234 has the environment variables:

• RANK=9
• LOCAL_RANK=0

When I configure Beyla to map the env var RANK to rank and LOCAL_RANK to local_rank
And I request the Beyla metrics endpoint
Then all metrics for PID 1234 should have rank=9, local_rank=0.

Note: OTEL_RESOURCE_ATTRIBUTES cannot be used as these processes are forked and assigned their env vars by a parent runner.