lua: add stats

[surki]

Commit Message: lua: add stats Additional Description: Currently stats are not available in lua filter, which makes it hard to track/alert for the script errors. This change adds error stat. Risk Level: Low Testing: Manual and Unit tests Docs Changes: Done Release Notes: Done Platform Specific Features: [Optional Runtime guard:] [Optional Fixes #Issue] [Optional Fixes commit #PR or SHA] [Optional Deprecated:] [Optional API Considerations:]

[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 @markdroth 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/22623 was opened by surki.

see: more, trace.

[kyessenov]

(re-assigning from @markdroth to @mattklein123 for API review)