Filter stats from stats/prometheus

[StrongMonkey]

Issue Template

Title: ability to filter stats from stats/prometheus

Description:
For /stats, there is a way to query(filter) stats based on regexp(adding ?filter). So I am wondering if there are similar expressions for stats/prometheus? I have a use case which I want to query stats from /stats/prometheus outside of envoy and would like to expect some query paramters that can be applied through the endpoint.

[mattklein123]

It looks like the filter regex is not passed to the prometheus handler. This should be a very simple change if someone would like to do a beginner PR.

envoy/source/server/http/admin.cc

Line 726 in c167c65

return handlerPrometheusStats(url, response_headers, response, admin_stream);

[StrongMonkey]

@mattklein123 thanks for the quick response! I ll put a pr

[mattklein123]

Fixed