Copy additional information as extensions of CE in APIServerSource

[danyinggu]

Helps with #3675

Proposed Changes

• Copy resource name, namespace and kind as extension attribute of CloudEvent in APIServerSource
  The CloudEvent has name, kind and namespace as attributes in the extension:

☁️  cloudevents.Event
Validation: valid
Context Attributes,
  specversion: 1.0
  type: dev.knative.apiserver.resource.update
  source: https://10.8.0.1:443
  subject: /apis/v1/namespaces/default/pods/busybox
  id: 6f0b8c62-7caa-4832-a73a-ea9cd76897e6
  time: 2020-07-28T15:44:55.796035817Z
  datacontenttype: application/json
Extensions,
  kind: Pod
  knativearrivaltime: 2020-07-28T15:44:55.796778825Z
  knativehistory: default-kne-trigger-kn-channel.default.svc.cluster.local
  name: busybox
  namespace: default
Data,

Release Note

The APIServerSource now set `name`, `kind` and `namespace` as extension attributes in the CloudEvent.

Docs

[danyinggu]

/assign @nachocano @rhuss

[danyinggu]

/assign @lionelvillard

[nachocano]

/lgtm
/approve

Can you check if there is an example in knative/docs regarding showing the produced event, and add the extension there as well?
Can you add in the description of this PR that ApiServerSource now sets the name of the resource as an extension attribute in the CloudEvent, or something like that?

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: danyinggu, nachocano

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/adapter/apiserver/events/events.go	95.7%	96.2%	0.5

[danyinggu]

/lgtm
/approve

Can you check if there is an example in knative/docs regarding showing the produced event, and add the extension there as well?
Can you add in the description of this PR that ApiServerSource now sets the name of the resource as an extension attribute in the CloudEvent, or something like that?

Sure. I just edited the description and release note of this PR. I will change the docs to add these additional attributes. Thanks for the reminder! :)

[nachocano]

/lgtm

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-knative-eventing-integration-tests		0/3

Failed non-flaky tests preventing automatic retry of pull-knative-eventing-integration-tests:

test/e2e.TestBrokerChannelFlowTriggerV1Beta1BrokerV1Beta1
test/e2e.TestBrokerChannelFlowTriggerV1Beta1BrokerV1Beta1/InMemoryChannel-messaging.knative.dev/v1

[danyinggu]

/retest