Sanitize metrics

[jakubdyszkiewicz]

Summary

Prometheus

Convertion from statsd to prometheus breaks when we have clusters with dots.
Example:
statsd:

cluster.backend.kuma-demo.svc_3001.assignment_stale: 0

prometheus

envoy_cluster_kuma_demo_svc_3001_assignment_stale{envoy_cluster_name="backend"}

There were several ways to fix it.

1. We could introduce ._. suffix to clusters so you can skip extra part on Statsd and then build proper regex for Prometheus.
2. We can replace every . in cluster name with _.

We picked second solution since it's less error prone. So the metric with this fix looks like following:
statsd:

cluster.backend_kuma-demo_svc_3001.assignment_stale: 0

prometheus

envoy_cluster_assignment_stale{envoy_cluster_name="backend_kuma-demo_svc_3001"}

StatsD

We also have to sanitize metrics for StatsD as : and | chars are illegal.
Official func to sanitize metrics looks like this https://github.com/statsd/statsd/blob/master/stats.js#L167

I slightly modified it changing the illegal characters always to _.

Example files
fix_configdump.txt
fix_prometheus.txt
fix_statsd.txt

For the cluster name. Instead of changing the name in every place I leveraged alt_stat_name field. I think this is less error prone since we reference cluster names in many places in the config.