Add filtering machanism

[fabxc]

We know of users of the existing integration that filter collected metrics for cost control.
The GA version should allow for the same kind of controls. Users could filter the data stored by the main Prometheus server but mutating their existing setup is not a good experience in this case.

The simplest approach is probably a repeated flag that allows to set Prometheus-style label matchers, which must all pass, e.g.

./stackdriver-prometheus-sidecar \
    --filter='job="kubelet"' \
    --filter='__name__!~="cadvisor.*"' \
    ...

[fabxc]

Fixed via #47