@Override
	public void describeTo(Description description) {
		description.appendText("an existing metric");
	}
A matcher that will assert that the {@code <metric type> list} shell command returns a list that contains the
expected metric.

@param <T> the type of metric sink fixture this is matching
@author Eric Bottard
@author Marius Bogoevici
spring-xd-shell/src/test/java/org/springframework/xd/shell/command/fixtures/MetricExistsMatcher.java