[logging] REST payloads are over-stringified and lose JSON structure

[vchudnov-g]

In the current version of #2270, the httpResponse.payload field is over-stringified: it's a single string with string-encoded JSON, rather than JSON object with the various fields. Since the response is coming back as JSON, we should ideally present that JSON, or at least recreate it from the response proto, without further stringifying it.

In other words, we're getting

  "httpResponse": {
    "payload": "{\n  \"content\": \"The hail in Wales falls mainly on the snails.\",\n  \"requestId\": \"some_value\",\n  \"severity\": 0,\n  \"otherRequestId\": \"\"\n}",
    "status": 200,
    // ...
    }

whereas it would be more useful to get

  "httpResponse": {
    "payload": {
      "content": "The hail in Wales falls mainly on the snails.",
      "requestId": "some_value",
      "severity": 0,
      "otherRequestId": "",
      },
    "status": 200,
   // ...
   }

[vchudnov-g]

This might be happening because when populate payload in our _LOGGER.debug() statements, we convert it to JSON, and then our standard logging handler converts the resulting string to JSON yet again. In that case, it's like running json.dumps(json.dumps({"alpaha":"beta"}).

So the fix, then, would be that when we populate payload in the the _LOGGER.debug() statements, we convert the content into a Python dict (instead of a JSON string), so that when our default handler calls json.dumps(), it is handled correctly.