Unified Service Tagging cleanup for KSM, global env, version

[JacksonDavenport]

What does this PR do? What is the motivation?

https://datadoghq.atlassian.net/browse/TEEP-6589

Three main changes to this doc relative to each commit:

1. The Kubernetes State Metrics (KSM) portion is no longer relevant as you don't have to do any specific KSM configurations since the move to KSM Core. The KSM Check will look for and use the labels automatically.
2. On the Kubernetes / SSI docs we do make note that you can just add a global tags config to set your span metadata. Have double checked/tested to confirm that this works for the default behavior. It is a common enough recommendation so makes sense to make more official.
3. The snippet on automatic version tagging was duplicated in all 3 container tabs (Kubernetes, Docker, ECS), so extracted it out into the single section to simplify and avoid drift between them. Otherwise content wise it was the same (plus some minor link fixes)

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

May look at further changes to UST in the SSI doc in the near future, but good to do this first pass of this doc. To make those future changes easier.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/jack.davenport/ust-containers-cleanup/getting_started/tagging/unified_service_tagging

[janine-c]

Looks great! Just some minor writing feedback and some questions about some things I thought we might want to clarify. Let me know if you have any questions or want to chat about any of my comments!

[janine-c]

Suggested change

	The `DD_ENV`, `DD_SERVICE`, and `DD_VERSION` environment variables are used by APM enabled applications. These environment variables can be set manually or by the [Kubernetes's downward API][2] as seen below. If you are using the Cluster Agent's [Admission Controller][1] to mutate your pods, those three environment variables are automatically injected to match your labels.
	The `DD_ENV`, `DD_SERVICE`, and `DD_VERSION` environment variables are used by APM-enabled applications. You can set these environment variables manually or using [Kubernetes's downward API][2] as seen below. If you are using the Cluster Agent's [Admission Controller][1] to mutate your pods, those three environment variables are automatically injected to match your labels.

This is a bit of a long page; can we add some links when we refer to things below, so we can help users get to the relevant information?

[JacksonDavenport]

Added your suggested changes separately for this, but wasn't quite sure if you were looking for something else regarding the links. Those two should resolve to the

• https://kubernetes.io/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/#capabilities-of-the-downward-api
• https://docs.datadoghq.com/containers/cluster_agent/admission_controller/?tab=datadogoperator#configure-apm-and-dogstatsd-communication-mode

Otherwise ready for re-review

[janine-c]

Excellent, thanks for the quick edit! One last thing I want to figure out and then we're good to go!

[janine-c]

I find the use of "following" here confusing, because we don't list the labels; we talk about environment variables instead. Can we clarify the relationship between them?

[janine-c]

Suggested change

	The `DD_ENV`, `DD_SERVICE`, and `DD_VERSION` environment variables are used by APM-enabled applications. You can set these environment variables manually or using the [Kubernetes's downward API][2] as seen below. If you are using the Cluster Agent's [Admission Controller][1] to mutate your pods, those three environment variables are automatically injected to match your labels.
	The `DD_ENV`, `DD_SERVICE`, and `DD_VERSION` environment variables are used by APM-enabled applications. You can set these environment variables manually or using the [Kubernetes's downward API][2]. If you are using the Cluster Agent's [Admission Controller][1] to mutate your pods, those three environment variables are automatically injected to match your labels.

Think we're good to remove this reference to "below" if we have a link!