Add a base_job_name tag to encompass all instances of a given job

[chrissnell]

When outputting Prometheus metrics, k6-operator includes a job_name tag, but the value of this tag corresponds to the individual instance (== Pod) running a test. So, for a job called test, the values of job_name look like test-1, test-2, etc.

This makes it very difficult to build a dashboard that displays all jobs running a particular test. We need a tag that provides the name of the job without any instance numbers.

This PR adds a base_job_name tag that contains only the job name.

In the example above, this PR adds a base_job_name tag with a value of test.

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[chrissnell]

People want test-wide tags:

https://community.grafana.com/t/k6-operator-pass-local-environment-variable-as-test-wide-tag/99639

People are struggling with bugs:

#162
#211