chromeos: Add --external-metrics-collection-interval.

Add a switch that can be used to supply an alternate
interval (in seconds) between the ExternalMetrics class
reading UMA events and histograms reported by Chrome OS
system daemons from /var/lib/metrics/uma-events.

I intend to use this in a test that will verify this
reporting path (without needing to wait the current default
of 30 seconds).

Bug: 878641
Change-Id: I83f3ba70b7c32ddd598e1793a71900ef5b72b667
Reviewed-on: https://chromium-review.googlesource.com/1202023
Commit-Queue: Dan Erat <[email protected]>
Reviewed-by: Achuith Bhandarkar <[email protected]>
Cr-Commit-Position: refs/heads/master@{#588991}
5 files changed