Retrieve device stats for docker containers

[robiscoding]

Description

• This is a rework of Add /device/stats endpoint #707. With the /device/stats endpoint, users can retrieve the results of the docker stats command in json format

Changed the functionality:

• changed IS_DOCKER flag which may be a misleading name into env variable to explicitly control the device stats to be enabled - such that people running on prem will not get the feature out of the box
• added docs describing feature
• now when you have misconfigured the service, you will be given 404 (when DOCKER_SOCKET_PATH is not exported) or HTTP 500 when exported value do not match docker socket.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How has this change been tested, please provide a testcase or example of how you tested the change?

• Tested with and without the socket and verified we get the error message when the socket isn't mounted and the actual json stats when it is

Any specific deployment considerations

For example, documentation changes, usability, usage/costs, secrets, etc.

Docs

• Docs updated? What were the changes: