PoC: object annotation trace context carrier

[dashpole]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Example implementation of context propagation using object annotations as the carrier. This would enable linking kubelet spans to spans from other controllers.

ExtractContext gets the context from an object (this will be in the w3c traceparent format).

InjectContext adds the context to an object.

Which issue(s) this PR fixes:

Special notes for your reviewer:

This is similar to ideas documented in kubernetes/enhancements#2312.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

@richabanker @logicalhan @tallclair @dgrisonnet

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[ffromani]

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: dashpole
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Needs approval from an approver in each of these files:

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• pkg/kubelet/OWNERS
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• staging/src/k8s.io/component-base/tracing/OWNERS [dashpole]

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[yharish991]

how is this context passed to ExtractContext available to all the controllers? is it some kind of global context?

[dashpole]

This accepts a context argument in-case the controller already has a context. Most of the time, controllers will probably use a context.Background(). But if the controller is already propagating a context (e.g. for passing metadata to contextual logs), this ensures the two can be merged together (since we can add to an existing context, rather than requiring a new one.