[monitoring] Device metric collection view should be a POST

[nemesifier]

This should be a POST and not a GET:

openwisp-monitoring/openwisp_monitoring/device/api/views.py

Line 38 in 9972037

def get(self, request, pk):

Tests have to be updated accordingly, devices will have to be updated as well.

Let's provide backward compatibility for the GET, but using logger.warning() to suggest to upgrade.

The lua monitoring scripts will also have to be upgraded: openwisp/openwrt-openwisp-monitoring#4

[nemesifier]

Working too much is unhealthy.. (It's a POST already.)