fix(auth): pass GRAFANA_TOKEN env variable on Grafana initialization

[jl2397]

When the Grafana class is initialized in grafana.py, there is only a check for the "url" field in the parameters. If the GRAFANA_TOKEN env variable is set, the token is never passed to the GrafanaApi client and therefore results in a GrafanaUnauthorizedError when making requests using grafana_client. This adds a credential field to the from_url call and populates it with the GRAFANA_TOKEN env variable.

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[amotl]

Thank you very much for your contribution. I've converged it into GH-20, adjusting slightly for additional code formatting.

NB: poe format would have solved it well.