[Monitor Ingestion] Improve docs and experience for users using JSON strings

[pvaneck]

A common scenario is that users might mistakenly try sending a JSON string to the LogsIngestionClient.upload method.

Some improvements we can implement are:

1. Add another check to see if the user is passing in a string and raise a ValueError if they are.
2. Include some documentation/code snippet in the readme for reading JSON strings and uploading the JSON data.