small improvements to metrics server example

[ryandawsonuk]

I noticed https://github.com/SeldonIO/seldon-core/tree/master/examples/feedback/metrics-server was using an old version of knative so I thought I would update it... then I realised it doesn't really use knative at all as the logger uri points directly at the metrics server.

So instead I removed knative from this particular example. If people want to see the knative flow they can use the simlar example https://github.com/SeldonIO/seldon-core/tree/master/examples/feedback/feedback-metrics-server (and have added a note saying this). This makes it a bit easier both to run and to maintain.

Have run the notebook through and all works (which makes sense as haven't really made a substantial change here).

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