orca: fix the delimiter of the native http parser

[wbpcode]

Commit Message: orca: fix the delimiter of the native http parser
Additional Description:

This should also be cherry-pick to 1.32 to fix this problem.

The previous ORCA parser will use : as the delimiter of key/value pair in the native HTTP report. This is wrong
based on the design document. The correct delimiter should be =. This change fixes the delimiter to =.
See #6614 and related documents for more details.

https://docs.google.com/document/d/1NSnK3346BkBo1JUU3I9I5NYYnaJZQPt8_Z_XCBCI3uA/edit?tab=t.0

Native HTTP header encoding.  Similar to the JSON dictionary format, we will define Endpoint-Load-Metrics as having 
key=value format and use standard HTTP header encoding to either send the metrics as a sequence of HTTP headers or 
as a single header with [comma separated](https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2) key-value 
pairs. E.g.  endpoint-load-metrics: TEXT cpu=0.3,mem=0.8,foo_bytes=123.

Risk Level: n/a.
Testing: n/a.
Docs Changes: n/a.
Release Notes: added.

[wbpcode]

cc @blake-snyder @efimki

[wbpcode]

This is a behavior change. But the orca feature should not be widely used.

I am thinking should we change it directly, or add a runtime flag, or to support both ":" and "=" as delimiters (although may not match the document). cc @alyssawilk

[wbpcode]

/retest

[adisuissa]

This change will probably be needed to be updated in many docs.
cc @AndresGuedez to weigh in.

[wbpcode]

This change will probably be needed to be updated in many docs. cc @AndresGuedez to weigh in.

I forget to push the change log. Actually, this PR fixed the implementation based on the design document. In the design document, = should be used. But in previous implementation, this : was mis-used. This PR correct that mistake.

[AndresGuedez]

Thanks for catching this, agreed the design calls for '=' as the delimiter. However, this PR is backwards incompatible for existing users, so supporting both ':' and '=' would be preferable IMO.

[wbpcode]

Thanks for catching this, agreed the design calls for '=' as the delimiter. However, this PR is backwards incompatible for existing users, so supporting both ':' and '=' would be preferable IMO.

Then we need to ensure both : and = will be treated as special character and won't be a part of metric name. Of course, I am ok to that.

[blake-snyder]

Nice catch. Thanks, @wbpcode !

[efimki]

Looks good to me, thank you for fixing this in backwards compatible way!

[wbpcode]

/retest

[adisuissa]

Thanks!
Small nit, but otherwise LGTM.

[adisuissa]

LGTM, thanks!