Avoid JSON encoding event payloads more than once

[imjoehaines]

Goal

Currently we JSON encode payloads three times:

1. once to check the payload size
2. once to re-check the payload size after possibly removing metadata — this is unconditional so happens even if the payload is already small enough
3. once using Guzzle's json option

This PR ensures that we only JSON encode payloads once when possible. If a payload is too large to send after encoding, we will still do a second encode after removing metadata