Distinguish different workers in Prometheus PushGateway

[CaptainIRS]

In this PR:

• The worker index of each worker is added as a label when pushing metrics to PushGateway to distinguish workers.

Fixes #1354

Results:

/metrics endpoint of PushGateway

Grafana dashboard showing metrics from two workers simultaneously

[davidkel]

With every code change in caliper we need to add unit tests, and so we need a new unit test for this change as I don't think there is a unit test that covers this code

[CaptainIRS]

I have looked at https://github.com/hyperledger/caliper/blob/main/packages/caliper-core/test/worker/tx-observers/prometheus-push-tx-observer.js, but there doesn't seem to be any tests covering _sendUpdate as it is being stubbed out in unit tests. I'm not sure how a unit test for this would look like as pushAdd is a method from the prom-client library. Should the unit test just ensure that workerIndex is being passed to the pushAdd method (by using mocks)?

[davidkel]

We would need a stub of the method this.prometheusPushGateway.pushAdd so we can track that it is being called and passing the expected parameters. This will then give us code coverage of that line and we know that it executes and passes the correct parameters, also checks that the call is likely to succeed and not fail when executed for real.

[davidkel]

LGTM