proxy_protocol_filter: Add field `stat_prefix` to the filter configuration

[nareddyt]

Commit Message: proxy_protocol_filter: Add field stat_prefix to the filter configuration

Additional Description: This field allows for differentiating statistics when multiple proxy protocol listener filters are configured.

This PR is a follow-up from previous conversation: https://github.com/envoyproxy/envoy/pull/32861#discussion_r1539832395

Risk Level: Low

• All client-facing behavior changes are guarded by new filter config field.

Testing:

• Stats unit tests
• Proxy protocol listener filter integration tests

Docs Changes: Done

Release Notes: Done

Platform Specific Features: None

[repokitteh-read-only[bot]]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/). envoyproxy/api-shepherds assignee is @wbpcode CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/34414 was opened by nareddyt.

see: more, trace.

[nareddyt]

@ggreenway I believe you have the most context based on https://github.com/envoyproxy/envoy/pull/32861#discussion_r1539832395. Sorry for the late follow-up!

[nareddyt]

PTAL @jmarantz

[nareddyt]

PTAL, would like to get this merged soon.

[nareddyt]

@jmarantz this should be a quick review :)

[jmarantz]

one documentation nit remains.

[nareddyt]

one documentation nit remains.

Done, PTAL

[nareddyt]

Looks like fuzz coverage is broken, seems unrelated to my PR

[nareddyt]

/retest

[nareddyt]

PTAL, CI passed @jmarantz @ggreenway

[nareddyt]

PTAL @jmarantz