Add -metrics-name-rewriting option to replace '.' with ':'

[dgl]

Without this we do this for requests but not responses, which we got away with because Thanos doesn't validate the metric names as closely. But Prometheus remote_read doesn't allow '.', so always replace the name with the ':' version.

An option for now because this is confusing behaviour:

• Matches on __name__ will use the '.' version, but return the ':' version. i.e. {__name__="tsd.rpc.recieved"} will return "tsd:rpc:recieved"

Fixes #79.

[eswdd]

Wondering if we can name the option to make it clear we're renaming on the response, as otherwise one might be forgiven for thinking that without it we don't rewrite "." to ":" when doing a name query