Collect GET request should indicate if underlying services are reachable

[kinghuang]

The response for GET requests unconditionally returns a 200 response with {"status": "GET ok"} right now. This should be changed to better indicate the health of the collector. For example, if Kafka isn't reachable, it could return something appropriate for that.

[iinuwa]

Hello @kinghuang! I'm interested in helping get this off the ground. What kind of checks do you think need to be implemented (besides Kafka being unreachable)?

[kinghuang]

Hi, @iinuwa! Thanks for your interest! I'm thinking of implementing the HEALTHCHECK instruction in the Docker image along with returning a non 200 response. Haven't had time to think more about what else is needed. We've used this successfully as is for an in-development system, internally.

This also reminds me, I should update the README!