[core] Support commit metrics

[schnappi17]

Purpose

Linked issue: close #1468

[schnappi17]

Remove total- metrics, which is more suitable to query system files table.

[tsreaper]

Suggested change

	try {
	assertThat(Metrics.getInstance().getMetricGroups().size()).isEqualTo(1);
	} catch (AssertionError e) {
	error =
	ExceptionUtils.firstOrSuppressed(
	e,
	new AssertionError(
	"Please close the created TableCommit objects in case of metrics resource leak."));
	}
	assertThat(Metrics.getInstance().getMetricGroups().size()).isEqualTo(1).withFailMessage(xxx);

Please close the created TableCommit objects -> Please close the created metric groups. In the future we're going to add more metrics aside from commit metrics.

You might want to add a helper method to print out all metric group objects, so developers can see what kinds of metric groups are not closed.