How to setup X-Forwarded-For HTTP header to preserve the original client IP address for traffic translated by a SNAT ?

just to clarify, you want to rewrite the list of IPs from the XFF header to be only the first IP in the list? If so, this should work for you.

when HTTP_REQUEST {
  HTTP::header replace X-Forwarded-For [getfield [HTTP::header X-Forwarded-For] "," 1]
}

KOR, the HTTP profile's Request Header insert field will not accomplish what you're looking for. It will not expand an iRule value like [IP::client_addr], but rather takes a static value (ex. TEST=Foo). Further, the Insert X-Forwarded-For option will explicitly insert an "X-Forwarded-For" header into the request. If you want a specific header name other that X-Forwarded-For, you would want to use an iRule:

when HTTP_REQUEST {
    HTTP::header insert X-BIGIP-CALLING-IP [IP::client_addr]
}

It looks like Google adds a new header to pass the original IP address from the client:

Forwarded: for=105.235.130.92

You may use an iRule to extract the IP address from that header and add it to the X-forwarded-for header.

is it something like this?