[prometheusreceiver] allow to pull Prometheus jobs from /job endpoint of the OpenTelemetryOperator-TargetAllocator

[secustor]

Is your feature request related to a problem? Please describe.
The TargetAllocator is component of a Collector deployment which allows to shard Prometheus targets on multiple Collector instances.

To allow the Collector to dynamically discover new jobs the /jobs endpoint of the TargetAllocator should be used.

Describe the solution you'd like

receivers:
    prometheus:
      target_allocator:
        endpoint:  http://my-targetallocator-service
        interval: 30s
        collector_id: ${COLLECTOR_ID}
# imported struct from the Prometheus code base. Can be used optionally to configure the jobs as seen in the docs 
# https://prometheus.io/docs/prometheus/latest/configuration/configuration/#http_sd_config
        http_sd_config: 
          basic_auth:
            username: prometheus
            password: changeme

With this configuration prometheusreceiver will scrape the /job endpoint every 30s to discover jobs.

These jobs will be then used to generate a scrape configuration with a HTTP_SD job per returned job.
Therefore if /job returns following JSON:

{
  "job1": {
    "_link": "/jobs/job1/targets"
  },
  "job2": {
    "_link": "/jobs/job2/targets"
  }
}

This scrape config will be generated and then applied:

scrape_configs:
- job_name: job1
  http_sd_configs:
  - url: "http://my-targetallocator-service/jobs/job1/targets?collector_id=aCollector"
    basic_auth:
        username: prometheus
        password: changeme
- job_name: job2
  http_sd_configs:
  - url: "http://my-targetallocator-service/jobs/job2/targets?collector_id=aCollector"
    basic_auth:
        username: prometheus
        password: changeme

Describe alternatives you've considered
Some alternatives have been considered, these can be found in this issue: open-telemetry/opentelemetry-operator#713

[Aneurysm9]

@secustor do you plan to work on this or should I look for someone with bandwidth to take it on?

[secustor]

Yes, I plan to implement this myself