APM agent crashes if the request body is too large

[Ofekroz10]

Describe the bug: ... When the request body is too large, reporting to the apm server fails. The error message is:

eapm event processor thread elasticapm.transport:286 - Failed to submit message: 'HTTP 400: {"accepted":4,"errors":[{"message":"event exceeded the permitted size." ...

To Reproduce

1. Check the body's size before reporting. If it's too large, report the body as [Too large] or something like that.

Environment (please complete the following information)

• OS: mac
• Python version: 3.7
• Framework and version [e.g. Django 2.1]: django==2.2.26
• APM Server version: ?
• Agent version: 5.10.1

Elastic apm configuration (in settings.py):

ELASTIC_APM = {
	...
    "CAPTURE_BODY": "all",
}

Additional context

The issue is that Django client extracts the request body without checking the size of the body. I believe that adding a size check in this function should solve the issue.

[basepi]

Fix open in #1610