fix(divide): account for header names and framing in headerMaxSize (#…

[harshkha3417]

Fixes #7818.

Motivation

In DividePlugin, headerMaxSize previously summed only the byte lengths of request header values:

long headerSize = exchange.getRequest().getHeaders().values()
        .stream()
        .flatMap(Collection::stream)
        .mapToLong(header -> header.getBytes(StandardCharsets.UTF_8).length)
        .sum();