filter state: add hashable string factory

[kyessenov]

Change-Id: Ide0862550ebd67fc0de034daaac42be8f0fd6688

Commit Message: add a hashable string that can influence HTTP connection pool when shared with upstream.
Additional Description:
Risk Level: low
Testing: yes
Docs Changes: yes
Release Notes: no

[agrawroh]

Nice! I remember that I followed your suggestion long back when we saw an issue with filter-state mixup in TCPProxy and had an internal patch (which I also tried upstreaming before) with almost the same implementation running internally.

It's awesome that we are adding this now as I was too scared to switch back to envoy.string after that outage and have the custom patch running all this time.

[kyessenov]

@agrawroh Yeah, I know it's a tricky configuration. Let me know how else I can improve it. I am worried that people are blindly copy-pasting configs from LLM without thinking about it, and this is quite subtle.

[agrawroh]

@agrawroh Yeah, I know it's a tricky configuration. Let me know how else I can improve it. I am worried that people are blindly copy-pasting configs from LLM without thinking about it, and this is quite subtle.

It looks good to me. Maybe just a warning or note in the docs on the performance penalty as compared to envoy.string would be good.

[agrawroh]

LGTM, Thanks @kyessenov.