Add back logging of incoming event for observability

[lanaebk]

Need to add back logging of incoming event to troubleshoot errors.

Example from today:

{
    "errorType": "TypeError",
    "errorMessage": "Cannot read property 'author' of null",
    "stack": [
        "TypeError: Cannot read property 'author' of null",
        "    at Object.exports.getMergeRequestEventData (\/var\/task\/src\/merge_request\/helpers.js:19:58)",
        "    at Object.<anonymous> (\/var\/task\/src\/bot_actions\/bot_action.js:101:51)",
        "    at Generator.next (<anonymous>)",
        "    at \/var\/task\/src\/bot_actions\/bot_action.js:8:71",
        "    at new Promise (<anonymous>)",
        "    at __awaiter (\/var\/task\/src\/bot_actions\/bot_action.js:4:12)",
        "    at Object.runBotActions (\/var\/task\/src\/bot_actions\/bot_action.js:100:12)",
        "    at \/var\/task\/handler.js:69:56",
        "    at Generator.next (<anonymous>)",
        "    at fulfilled (\/var\/task\/handler.js:5:58)"
    ]
}