cloud_run_alerts: add flexible json-based log indicators and policies

[andriipetruk]

This PR enhances the cloud_run_service_alerts module by adding support for flexible JSON-based log indicators and alert policies:

• JSON Payload Labels: Easily extract custom labels from JSON logs.
• Flexible Alert Policies: Now supports defining multiple metrics within a single alert policy.

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[andriipetruk]

This PR enhances the cloud_run_service_alerts module by adding support for flexible JSON-based log indicators and alert policies:

• JSON Payload Labels: Easily extract custom labels from JSON logs.
• Flexible Alert Policies: Now supports defining multiple metrics within a single alert policy.

[pjh]

I've never looked at this code before today but on the whole this change looks good to me. I reviewed it to try to understand if / how it would affect existing users of the module and for the most part it seems to preserve the existing behavior, though I did have questions in one or two parts.

I know that @shankiyani has started to review this and is perhaps testing it out in his environment, I'll defer approval to him.

[andriipetruk]

@shankiyani @pjh PR was updated

[pjh]

@shankiyani please re-review, I'm deferring approval to you.

[shankiyani]

@shankiyani @pjh PR was updated

Thanks @andriipetruk setting up a local test will take some time and I want to think through if this pattern should be adopted for the text payload variant as well.

In general multi-condition alert policy hinders the ability to use Cloud Monitoring Dashboards with no indicator that the feature will be supported anytime soon. I've been wanting to create a more "flattened" approach at the cost of increased number of APs but the draw being that dashboards can be used for centralized on-call viewing. Its not blocking for this PR but I wanted to share my intentions for this module going forward.