docs: Highlight how a format string can be used as a value for a header

[arkodg]

Description:

Describe the issue.

Envoy allows you to use format strings https://www.envoyproxy.io/docs/envoy/latest/configuration/observability/access_log/usage#config-access-log-format-strings which can use set as header values.

here's an example where it can used to set the downstream addr value in the X-Proxy-Connection header.

    - filters:
        - type: URLRewrite
          urlRewrite:
            path:
              type: ReplacePrefixMatch
              replacePrefixMatch: /
        - type: RequestHeaderModifier
          requestHeaderModifier:
            set:
              - name: X-Proxy-Connection
                value: "%DOWNSTREAM_REMOTE_ADDRES%"

Would be great to add a similar example where users can access traffic attributes using this technique

and add this into https://gateway.envoyproxy.io/docs/tasks/traffic/http-request-headers/

[optional Relevant Links:]

Any extra documentation required to understand the issue.

[mahdikhashan]

@arkodg hey, i'd like to work on this issue as my first contribution to the project - would you please assign it to me to prevent duplicate work?

[github-actions]

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