Introduce interval field for kubectl get KeptnMetric

[rakshitgondwal]

With the introduction of new range.interval field in the KeptnMetric, it is now possible to query metrics over a timeframe (see #1471 for more info).

It would be a good idea to include an interval field for kubectl get KeptnMetric.

• This field should contain the value of range.interval.
• If range.interval is empty, the field should be empty too.