Add support for custom Envoy JSON fields

[mike1808]

Add support for custom Envoy JSON fields via = in the fields name. The specify custom field name and its Envoy format string use = in the field name. The syntax is the following: field_name=Envoy format string, where Envoy format string can have any command except DYNAMIC_METADATA and FILTER_STATE commands.

Example of custom field:

json-fields:
  - @timestamp
  - customer_id=%REQ(X-CUSTOMER-ID)%

Fixes #3032, Fixes #1507

cc @KauzClay

[codecov]

Codecov Report

Merging #3059 into main will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3059      +/-   ##
==========================================
- Coverage   73.22%   73.21%   -0.01%     
==========================================
  Files          95       95              
  Lines        6057     6056       -1     
==========================================
- Hits         4435     4434       -1     
  Misses       1521     1521              
  Partials      101      101              

Impacted Files	Coverage Δ
internal/xdscache/v2/listener.go	95.10% <ø> (ø)
internal/envoy/v2/accesslog.go	100.00% <100.00%> (ø)

[jpeach]

Hi @mike1808,

Overally, I think this looks pretty good.

1. We should document this in ./site/_site/docs/main/configuration.md. It's
   OK to do this in a separate PR if you prefer.

2. I think that it would be clearer to split the validation regex to handle the
   simple case (e.g. %HOSTNAME%) and complex case (e.g. %REQ(foo?bar):baz%)
   separately. This would make the regex much more obvious and the code easier to
   follow. The REQ/RESP regex only needs to validate the capture from the simple
   case (i.e. REQ(foo?bar):baz)

3. Once you do (2), there's an opportunity to have a single definition of the
   simple expressions. If you define EnvoyOperators to just be the simple
   operators, then you can derive the JSON field name from that, and add those
   mappings to JSONFields in an init function. The EnvoyOperators map should
   go into pkg/config/accesslog.go.

[mike1808]

@jpeach

1. We'll add the docs in the separate PR.
2. I don't think that's possible because Envoy can have multiple operators in one field. E.g. it can have a simple operator like %HOSTNAME% and parameterized one like %REQ()%. like req from %HOSTNAME% with %REQ(X-CONTNENT-ID)%
3. Because the option (2) is not possible we cannot do that. But we can move EnvoyOperators to pkg/config/accesslog.go

[mike1808]

@jpeach we updated the PR and tried to address all review comments. One thing we're not sure about is our approach for keeping parsed access log fields in the Parameters structure.

cc @XanderStrike

[mike1808]

@jpeach we renamed JSONFields to jsonFields and encapsulated all the translation logic in config package.

[youngnick]

LGTM, nice work. I'm looking forward to seeing the docs update as well, I think that's key for making this feature more usable. We should wait for @jpeach to approve as well though.

[jpeach]

Thanks @mike1808, this looks great. Could you just take care of the couple of nits, then we can merge.

[XanderStrike]

Latest comments addressed and fixed up into the commit, rebased against projectcontour:main. Ready for review again.