Explicit tagging feature

[xlucas]

What does this PR do?

It introduces pod-level and container-level tagging through annotations.

Motivation

Being able to configure per-container and per-pod tags with a more dynamic approach (so we can dispense with rollouts implied by label or configuration change).

Example

The intended usage is:

metadata:
  annotations:
    ad.datadoghq.com/tags: {"pod_tag": "value", ...}
    ad.datadoghq.com/<container_name>.tags: {"container_tag": "value", ...}

[bits-bot]

All committers have signed the CLA.

[CharlyF]

Logic looks good to me 👍. @l0k0ms This should be in 6.10 - We'll need to sync to update the doc.
I think this deserves a relnote though.

[codecov]

Codecov Report

Merging #3024 into master will increase coverage by 0.03%.
The diff coverage is 74.07%.

@@            Coverage Diff             @@
##           master    #3024      +/-   ##
==========================================
+ Coverage   53.42%   53.45%   +0.03%     
==========================================
  Files         532      530       -2     
  Lines       37811    37948     +137     
==========================================
+ Hits        20199    20284      +85     
- Misses      16403    16460      +57     
+ Partials     1209     1204       -5

Impacted Files	Coverage Δ
pkg/tagger/collectors/kubelet_extract.go	78.76% <74.07%> (-1.48%)	⬇️
pkg/secrets/check_right.go	26.31% <0%> (-45.12%)	⬇️
pkg/secrets/secrets.go	63.1% <0%> (-8.05%)	⬇️
pkg/forwarder/transaction.go	78.63% <0%> (-4.28%)	⬇️
pkg/autodiscovery/listeners/kube_services.go	34.4% <0%> (-2.81%)	⬇️
pkg/autodiscovery/config_poller.go	30.23% <0%> (-1.87%)	⬇️
pkg/secrets/fetch_secret.go	96.36% <0%> (-0.07%)	⬇️
pkg/util/kubernetes/hpa/hpa.go	100% <0%> (ø)	⬆️
...kg/tagger/collectors/kubernetes_metadata_mapper.go	0% <0%> (ø)	⬆️
pkg/clusteragent/custommetrics/provider.go	61.9% <0%> (ø)	⬆️
... and 8 more

[xlucas]

Relnote added @CharlyF

[CharlyF]

🎖 🛫