prometheus: check return value from CheckpointSet.ForEach

[oncilla]

This PR modifies prometheus.Collect to reflect the change introduced
by #557.

The export{Counter,Histogram,LastValue,Summary} methods now all return
an error instead of calling the error callback directly.
The callback is now only called on the returned error from ForEach.

fixes #563

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This change is 

[paivagustavo]

LGTM.

For a next step I would love to see us using the go 1.13 error wrapping, I think Josh already mentioned this somewhere.

[oncilla]

@paivagustavo I would be happy to pick that up. (In a separate PR, I guess)
I assume you just want to have additional context.
E.g.

	lv, _, err := lvagg.LastValue()
	if err != nil {
		return err
	}

would turn into something like:

	lv, _, err := lvagg.LastValue()
	if err != nil {
		return fmt.Errorf("error retrieving last value: %w", err)
	}

[paivagustavo]

Exactly like that @oncilla and indeed in another PR :)