AssignMessage - How can I get all headers ?

There is no way to read a single variable that contains, for example, all of the names and values of the inbound headers in a JSON format. If you want to embed the names and values of the inbound request headers into the payload of a new, different request, then you probably want to use a JavaScript callout to do so.

If I use this JavaScript in the response flow:

var rh = context.getVariable('request.headers.names');
context.proxyResponse.content = String(rh);

...then when invoking the API Proxy I see this result:

$ curl -i https://$ORG-$ENV.apigee.net/jstest-1/t1
HTTP/1.1 200 OK
Date: Tue, 21 Jan 2020 22:42:45 GMT
Content-Type: text/plain
Content-Length: 80
Connection: keep-alive


[Accept, Host, User-Agent, X-Forwarded-For, X-Forwarded-Port, X-Forwarded-Proto]

What you're seeing there is the response payload contains a stringified version of the list of request header NAMES.

If instead I use this Javascript:

var rh = context.getVariable('request.headers.names');
var s = String(rh);
// convert the list of header names to an array:
var list = s.substring(1, s.length - 1).split(new RegExp(', ', 'g'));
var hash = {};
list.forEach(function(headerName) {
  hash[headerName] = context.getVariable('request.header.' + headerName);
});
context.proxyResponse.content = JSON.stringify(hash, null, 2);

Then the response payload is more like this:

{
    "Accept": "*/*",
    "Host": "gaccelerate3-test.apigee.net",
    "User-Agent": "curl/7.61.1",
    "X-Forwarded-For": "54.14.13.16",
    "X-Forwarded-Port": "443",
    "X-Forwarded-Proto": "https"
}

AFAIK you cannot do that with AssignMessage.