Add a receive handler to Telemeter for remote-write

[metalmatze]

We want to add a remote-write receive handler to Telemeter.
For this we need to add a new authorizer, as we are unable to use JWTs.

Works with a mock authorization-server.
Here's a Prometheus config to test with:

global:
  scrape_interval:     15s
  evaluation_interval: 15s

scrape_configs:
  - job_name: 'prometheus'
    static_configs:
    - targets: ['localhost:9090']
  - job_name: 'telemeter'
    static_configs:
    - targets: ['localhost:9004']

remote_write:
  - url: "http://localhost:9003/metrics/v1/receive"
    bearer_token: 'eyJjbHVzdGVyIjogImU1MGFiNGIwLTgwZjgtNDU3OC05NWVmLWQ0NzI5Njg5NzZhZSIsICJ0b2tlbiI6ICJhIn0K'

TODOs:

• Add some basic metrics (compare with forward store)

/cc @squat @brancz @kakkoyun

[brancz]

This'll work for now, but doesn't generally work. We need to rethink this a bit. Not a blocker on this though.

[squat]

Ack, this is 100% specific to Tollbooth. We’ll need to figure out a way to generalize this and maintain compat with Tollbooth auth

[squat]

I just realized that there is a decent way we can keep this generic and not tightly coupled to tollbooth. tollbooth expects all authorization requests to be POST requests with the following body:

{"cluster_id":"<cluster_id>","authorization_token":"<pull_secret_auth_token>"}

We can ensure that the bearer token that prometheus is configured to send matches that format and then we POST that token to the specified URL. This way we don't have to do any custom marshalling/unmarshalling. The only variables then are the authorization URL and the token contents.

The one thing that is funny is that we are taking the bearer token header and using that as the body to of a POST request, but I still think that's better and more generic than what we have today.

[metalmatze]

Let's make 32*1024 a constant. :)

[squat]

/lgtm

[openshift-ci-robot]

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[squat]

/retest

[openshift-bot]

/retest

Please review the full test history for this PR and help us cut down flakes.