Support correct HTTP methods on core API endpoints

[dlvenable]

Is your feature request related to a problem? Please describe.

All HTTP methods work on all the core APIs (shutdown, list, metrics).

Describe the solution you'd like

Only the following HTTP combinations should be supported:

Path	Method
shutdown	POST
list	GET
list	POST
metrics/sys	GET
metrics/sys	POST
metrics/prometheus	GET
metrics/prometheus	POST