metrics: reduce cadvisor cardinality

[cirocosta]

Hey,

At the moment, there are a bunch of measurements arriving from cadvisor that simply don't matter for us:

• measurements from namespaces that we don't care about (e.g., kube-system stuff)
• metrics that are not valuable for us (I'm pretty sure we filter out a bunch)

To do so, we could introduce metric_relabel_configs:

# drop any metrics from the `cadvisor` job that
# are not regarding the namespace that we want (`test`)
#
          metric_relabel_configs:
            - source_labels: [namespace]
              regex: 'test'
              action: keep

Thanks!

[cirocosta]

done https://github.com/concourse/hush-house/commit/194876b45cd0acc0c22f3c29756ffa8caa71b36e

the approach taken was the same as proposed: whitelist the namespaces that we care about