Support both zipkin and jaeger clients

[Kelvin Mungai]

Is your feature request related to a problem? Please describe.
For instrumentation, it would be great to have the option to send out traces using either the zipkin or jaeger client

Describe the solution you'd like
COS currently ships with a jaeger client as the only tracing option, instead we would update the service to house both the jaeger and zipkin clients and use env vars to enable either of them
COS_JAEGER_ENABLED
COS_ZIPKIN_ENABLED