Modify the metrics controller to support multiple metric values

[rakshitgondwal]

Description

Once the new fields have been added to the KeptnMetric to support multiple metric values, we'll need to update the metrics-controller to support these new fields.

The metrics-controller would query the SLI provider in an interval of fetchIntervalSeconds and keep the last spec.range.storedResults amount of results. It basically acts as an LRU with a capacity of spec.range.storedResults.
The maximum amount should be tested and found as part of this ticket - a good value could be 255.

Later, we can modify the status field to have a status.results field that would contain a slice/array of a Result struct.
The Result struct can have the value, lastUpdated, err fields for a particular metric.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be
closed if no further activity occurs. Thank you for your contributions.

[mowies]

@rakshitgondwal any updates? I see the PR is still in draft mode

[rakshitgondwal]

Got busy with other stuff, will continue working on this @mowies