Add metric about metrics freshness

[olivierlemasle]

Add a metric custom_metrics_apiserver_metric_freshness_seconds, similar to the metric metrics_server_api_metric_freshness_seconds from metrics-server.

This metric can optionally be registered using RegisterMetrics(registrationFunc func(metrics.Registerable) error) error, e.g:

metrics.RegisterMetrics(legacyregistry.Register)

Also, in test-adapter:

• register the new metric custom_metrics_apiserver_metric_freshness_seconds;
• currently, the timestamp for custom metrics values is the time when the metric is queried. With this change, the timestamp used is the time when the metric value is created / updated.

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[olivierlemasle]

I've updated the PR to follow your suggestions.

[dgrisonnet]

Changes related to the new metrics looks good to me, but the ones in the test-adapter provider need to be reflected in https://github.com/kubernetes-sigs/custom-metrics-apiserver/blob/master/docs/getting-started.md. That can be done in a follow-up but we need to make sure that we keep the doc up-to-date.

/lgtm
/approve

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[olivierlemasle]

... but the ones in the test-adapter provider need to be reflected in https://github.com/kubernetes-sigs/custom-metrics-apiserver/blob/master/docs/getting-started.md. That can be done in a follow-up but we need to make sure that we keep the doc up-to-date.

Yes, I'm thinking about it since #108. I've just opened an issue for that topic: #132.

[zroubalik]

@olivierlemasle @dgrisonnet would be nice to have all exposed metrics documented somewhere.

[dgrisonnet]

I opened an issue to keep track of that: #134