Better API for querying metrics

I just want to read a metric :-).

Can we have a better API than this:

Iterable\> seenMetrics = job.metrics()
.queryMetrics(
MetricsFilter.builder()
.addNameFilter(MetricNameFilter.named("XX", "YY"))
.build())
.counters();
long seenSentinels = Iterables.isEmpty(seenMetrics) ? 0
: Iterables.getFirst(seenMetrics, null).committed();

This is very clunky :-P.

Ideally I'd like to read a metric with a name, and provide a default value if the metric is not there.

Imported from Jira [BEAM-2046](https://issues.apache.org/jira/browse/BEAM-2046). Original Jira may contain additional context.
Reported by: malo@google.com.

The issue names no repository files, tests, or entry points. Start by locating the Java metrics API behind job.metrics() and queryMetrics(), then review existing metric access patterns; done means an agreed API can read a named metric and return a default when it is absent.

Written by the indexing model from the issue text.