[metrics] Add a flag to set a default metric level
[metrics] Add a flag to set a default metric level

This patch adds the `metrics_default_level` flag, set to the lowest level
by default. The flag defines the default severity level to use when
filtering the metrics in the '/metrics' endpoint.

I am adding this flag to better support existing monitoring deployments
using the plain/parameterless URL when collecting metrics. Even
though the metric level can be filtered by using the URL query
parameters, existing deployments may not support configuring or
adjusting that URL.

Change-Id: I296fda7a6fefb952f1522bba3c1ff9804d5ce5cc
Reviewed-on: http://gerrit.cloudera.org:8080/14922
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu