compressor: add weaken_etag_on_compress to weaken strong ETags when compressing

[rafaelgaspar]

Generative AI Disclaimer: This PR was prepared with assistance from generative AI (Cursor). The implementation, tests, and documentation have been fully reviewed by me as the PR author, and I take responsibility for the changes.

Commit Message:
compressor: add weaken_etag_on_compress to weaken strong ETags when compressing

Additional Description:
Adds a new option weaken_etag_on_compress to the compressor filter's
response_direction_config. When enabled, strong ETag response headers
are weakened by prepending W/ to the value (e.g. "abc123" becomes
W/"abc123") instead of being removed when compression is applied. Weak
ETags (already starting with W/) are left unchanged.

This allows caches and conditional requests to keep working while indicating
that the representation was modified by compression, matching behavior used in
other proxies (e.g. Varnish). The default remains false, preserving
existing behavior (strong ETags are removed when compressing).

Fixes #35152

Risk Level: Low

Testing:
New unit tests in compressor_filter_test.cc:

• WeakenEtagOnCompressStrongEtag – strong ETag is weakened when compressing
• WeakenEtagOnCompressStrongEtagWithQuotes – strong ETag with quotes is weakened
• WeakenEtagOnCompressWeakEtagUnchanged – weak ETag is preserved
• WeakenEtagOnCompressWithStatusHeaderEnabled – weaken behavior with status header enabled

Docs Changes: Yes

• Updated docs/root/configuration/http/http_filters/compressor_filter.rst with an "ETag handling" section describing disable_on_etag_header and weaken_etag_on_compress, and added a YAML example for weaken_etag_on_compress.
• Updated "When response compression is applied" and compression-skip conditions to reference ETag handling.
• Clarified EtagNotAllowed status and not_compressed_etag stat descriptions.

Release Notes: Yes

• Added entry under new_features in changelogs/current.yaml for the compressor filter's weaken_etag_on_compress option.

Platform Specific Features: N/A

[repokitteh-read-only]

Hi @rafaelgaspar, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #43755 was opened by rafaelgaspar.

see: more, trace.

[repokitteh-read-only]

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

🐱

Caused by: #43755 was opened by rafaelgaspar.

see: more, trace.

[agrawroh]

cc @adisuissa Gentle ping for the review.

[agrawroh]

@rafaelgaspar It also needs a main merge.

/wait

[adisuissa]

Thanks! left a couple of high-level API comments.

[adisuissa]

Please move next to the disable_on_etag_header field.

[adisuissa]

If the fields weaken_etag_on_compress and disable_on_etag_header are conflicting (cannot set both to true), then the suggested API flow will be prioritizing the new field over the old one in the case both are set.
The reason is to support gradual rollout of the feature support to clients and servers.

[rafaelgaspar]

@adisuissa both suggestions have been incorporated into the code, let me know if there is anything else.

[paul-r-gall]

@adisuissa re-ping for "stalled" bit.