Add storage "middleware" for OpenTracing and Instrumentation

[tomwilkie]

The individual storage backends expose they're own, inconsistent metrics. I'm not suggesting these go away, as they have different retires policies etc, but a consistent wrapper for the Storage interface that adds instrumentation and tracing would also be useful.

[gouthamve]

This is a good example of how we did it in the caching layer, and can be used here: https://github.com/cortexproject/cortex/blob/master/pkg/chunk/cache/instrumented.go and https://github.com/cortexproject/cortex/blob/master/pkg/ring/kv/metrics.go

We'd need it for these interfaces: https://github.com/cortexproject/cortex/blob/master/pkg/chunk/storage_client.go#L5-L28

[adnxn]

looking at this first

[bboreham]

I took off 'good first issue' because the 'chunks' code is now deprecated and the 'blocks' code comes from Thanos project, so any changes would have to be made there first.

[friedrichg]

Distributed tracing is provided by opentelemetry now