Add OpenAPI definition for traffic metrics

[adleong]

This addresses #132 for the traffic-metrics API.

We define an OpenAPI 3.0.0 specification for the traffic metrics API. This formalizes the description of the API given in traffic-metrics.md.

I tested this specification by creating a fork of the smi-metrics controller and added kin-openapi to validate the requests and responses against the given specification. Modulo some version differences (smi-metrics is still on version v1alpha1) the validations passed.

Signed-off-by: Alex Leong [email protected]