Collection of custom metrics

[anushree-n]

Based on #787
Adds Implementation of Collect() method of Collector interface

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[anushree-n]

I signed it.

[vmarmol]

Lets rebase this off of master since we merged the other PR

[anushree-n]

This can be skipped if we want to continue collection of other valid metrics.

[vmarmol]

We should probably keep a slice of errors that we then compile together. We try to get all the metrics we can and only return errors at the end. We can use:

https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/util/errors/errors.go#L31

[rjnagal]

Can't we rely on the config to know if we are expecting floats or ints?

[vmarmol]

+1

[anushree-n]

My concern was the way in which the metric would be expressed : int, integer, int32, number ...
But yes, I think this is a better approach. We can use enum for metric units if necessary

[rjnagal]

@anushree-n Can you rebase and squash?

[anushree-n]

Done, @rjnagal :)

[rjnagal]

LGTM