Make scrape_interval configurable

[przemeklal]

Enhancement Proposal

COS Proxy supports grafana-agent cos-agent relation however we still cannot use it since the default scrape_interval in grafana-agent is 1m. In our deployments we always use the following scrape_interval values to scrape cos-proxies:

• 5m for cos-proxy related to NRPE units using the :monitors relation
• 15s for cos-proxy related to apps providing :dashboards and :prometheus-target

Without this feature, we'll be unable to adopt this new cos-proxy:cos-agent relation in our deployments.

[simskij]

Thanks! #75 also concerns this, but now we have another reason to do so.