feat: add HTML response handling for target allocator

[CharlieTLe]

Description:
Adds support for HTML output in the target allocator.

Link to tracking Issue(s):

• Resolves: UI for Jobs and Scrape Configs in Target Allocator Server #3622

Testing:

Configured unit tests with golden files to test that HTML renders the correct response.

To start the test, run

make prepare-e2e

Setup a TA

# Sets up ClusterRole with full access to all resources and nonResourceURLs
# This is just for demonstration purposes and should not be used in production
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: ta
rules:
- apiGroups: [ '*' ]
  resources: [ '*' ]
  verbs: [ '*' ]
- nonResourceURLs: [ '*' ]
  verbs: [ '*' ]
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: ta
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: ta
subjects:
  - kind: ServiceAccount
    name: ta-targetallocator
    namespace: default
---
# Sets up the TargetAllocator
apiVersion: opentelemetry.io/v1alpha1
kind: TargetAllocator
metadata:
  name: ta
  namespace: default
spec:
  # Sets up the ServiceMonitor for the TA
  observability:
    metrics:
      enableMetrics: true
  # Set up watching of ServiceMonitors
  prometheusCR:
    enabled: true
    serviceMonitorSelector: {}

Port-forward to the TA pod.

kubectl port-forward -n default deploy/ta-targetallocator 8080:8080

Once port-forwarded, go to http://localhost:8080.

Screenshots:

Home

Scrape Configs

Jobs

Targets

Collector

Job

Target

[atoulme]

Changes look ok to me.

[jaronoff97]

other than the path, i think everything looks great! thank you 🙇

[chenlujjj]

Is it ready to be merged?

[jaronoff97]

@chenlujjj there's one conflict, otherwise yes

[swiatekm]

I'm ok with merging this PR as-is, although I'd love it if the raw html could be removed from handler functions completely. Nonetheless, thank you for your work on this feature, it must've been a huge pain.