docs: ScrapeConfig proposal (#5279)

* docs: ScrapeConfig proposal
* Update Documentation/proposals/202212-scrape-config.md
---------

Co-authored-by: Joao Marcal <joao.marcal12@gmail.com>
Co-authored-by: Jayapriya Pai <slashpai9@gmail.com>
Co-authored-by: Simon Pasquier <spasquie@redhat.com>

Documentation/proposals/202212-scrape-config.md

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+# ScrapeConfig CRD
+
+* Owners:
+  * [xiu](https://github.com/xiu)
+* Related Tickets:
+  * [#2787](https://github.com/prometheus-operator/prometheus-operator/issues/2787)
+  * [#3447](https://github.com/prometheus-operator/prometheus-operator/issues/3447)
+* Other docs:
+  * n/a
+
+This document aims at creating a lower level `ScrapeConfig` Custom Resource Definition that defines additional scrape
+configurations the Kubernetes way.
+
+# Why
+
+prometheus-operator misses a way to scrape external targets using CRD. Users have either been abusing the Probe CRD
+(#3447) or additionalScrapeConfig to do so. Multiple use cases have been reported:
+
+* A user reported in the contributor office hours that their team serves Prometheus as a service to several teams across
+  multiple regions. These teams have tens of thousands of exporters running outside Kubernetes. To scrape them, each
+  team has to involve this user's team which makes it a bottleneck. While there is CI in place, errors happen and can
+  render Prometheus' configuration to be invalid. They would like to use a CRD to help the team serve themselves in
+  adding scrape configurations.
+* [@auligh in #2787](https://github.com/prometheus-operator/prometheus-operator/issues/2787#issuecomment-539568397)
+  reported needing a specific scrape configuration to scrape services running on the kubernetes nodes but don't have a
+  Service attached to them. This user wants to deploy these scrape configurations alongside the applications rather than
+  in a centralized way with `additionalScrapeConfigs`.
+* [@bgagnon in #2787](https://github.com/prometheus-operator/prometheus-operator/issues/2787#issuecomment-545510764)
+  mentions that `additionalScrapeConfigs`'s usage is error-prone as with usage, multiple unrelated scrape configurations
+  are bundled together.
+
+Furthermore, currently, there is a lot of code duplication due to the operator supporting several CRDs that generate
+scrape configurations. With the new `ScrapeConfig` CRD, it would be possible to consolidate some of that logic, where
+the other `*Monitor` CRDs could be migrated so that they create a ScrapeConfig resource that would ultimately be used by
+the operator to generate scrape configuration.
+
+## Pitfalls of the current solution
+
+Using `additionalScrapeConfig` comes with drawbacks:
+
+* Teams have to build an infrastructure to add scrape rules in a centralized manner, which creates a bottleneck since a
+  single team becomes responsible for the configuration
+* There is no input validation, which can lead to an invalid prometheus configuration
+
+# Goals
+
+* Provide a way for users to self-service adding scrape targets
+* Consolidate the scrape configuration generation logic in a central point for other resources to use
+
+## Audience
+
+* Users who serve Prometheus as a service and want to have their customers autonomous in defining scrape configs
+* Users who want to manage scrape configs the same way as for services running within the Kubernetes cluster
+* Users who want a supported Kubernetes way of scraping targets outside the Kubernetes cluster
+
+# Non-Goals
+
+* This proposal doesn't aim at covering all the fields in
+  [`<scrape_config>`](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config).
+  Specifically, it focuses first on `static_configs`, `file_sd_configs` and `http_sd_configs`.
+* Refactoring of the other CRDs is not in scope for the first version
+
+# How
+
+As described by
+[@aulig in #2787](https://github.com/prometheus-operator/prometheus-operator/issues/2787#issuecomment-559776221), we
+will create a new ScrapeConfig CRD, this new CRD will act the same as the other CRDs and append scrape configurations to
+the configuration. Usage of ScrapeConfig doesn't exclude the use of the other CRDs, they are not mutually exclusive.
+`ScrapeConfig` will allow for any scraping configuration, while the other CRDs provide sane defaults. This will allow
+for isolated testing of the new `ScrapeConfig` CRD.
+
+```mermaid
+graph TD;
+ServiceMonitor --> prometheusConfig
+PodMonitor --> prometheusConfig
+ScrapeConfig --> prometheusConfig
+prometheusConfig
+```
+
+Using a pseudo custom resource definition, we should have the following:
+
+```yaml
+apiVersion: monitoring.coreos.com/v1alpha1
+kind: ScrapeConfig
+metadata:
+  name: my-scrape-config
+  namespace: system-monitoring
+  labels:
+    test: value
+spec:
+  staticConfigs:
+    - <staticConfig>[] # new resource
+  fileSDConfigs:
+    - <fileSDConfig>[] # new resource
+  httpSDConfigs:
+    - <httpSDConfig>[] # new resource
+  relabelings: # relabel_configs
+    - <RelabelConfig>[] # https://github.com/prometheus-operator/prometheus-operator/blob/e4e27052f57040f073c6c1e4aedaecaaec77d170/pkg/apis/monitoring/v1/types.go#L1150
+  metricsPath: /metrics
+```
+
+with the following new resources:
+
+* `staticConfig`
+
+```yaml
+targets:
+  - target:9100
+labels:
+  labelA: placeholder
+```
+
+* `fileSDConfig`:
+
+```yaml
+files:
+  # Files here are string referencing a file existing in the Prometheus Pod. prometheus-operator is not responsible for
+  # these SD files. The operator should use Prometheus.ConfigMaps to mount these files in the pods and have them usable
+  # by ScrapeConfig. No validation on the content of the SD files is expected from prometheus-operator.
+  - /etc/prometheus/configmaps/inventory/file.json
+refreshInterval: 5m
+```
+
+* `httpSDConfig`:
+
+```yaml
+url: http://localhost:1234
+refreshInterval: 60s
+```
+
+This example doesn't list all the fields that are offered by prometheus. The implementation of all the fields will be
+done in an iterative process and as such, the expectation is not for all of them to be implemented in the first version.
+
+Also, to help selecting `ScrapeConfig`, a new field will be added to the Prometheus CRD, same as for `ServiceMonitor`,
+`PodMonitor` and `Probe` objects:
+
+```yaml
+[...]
+spec:
+  scrapeConfigSelector: ...
+  scrapeConfigNamespaceSelector: ...
+```
+
+Once the CRD is released, we will start refactoring the other CRDs. Since `ScrapeConfig` will allow for any
+configuration, it can also generate scrape configuration for the other CRDs.
+
+```mermaid
+graph TD;
+ServiceMonitor --> ScrapeConfig
+PodMonitor --> ScrapeConfig
+ScrapeConfig --> prometheusConfig
+prometheusConfig
+```
+
+# Alternatives
+
+* Use `additionalScrapeConfig` secrets, with the pitfalls described earlier
+
+# Action Plan
+
+1. Create the `ScrapeConfig` CRD, covering `file_sd_configs`, `static_configs` and `http_sd_configs`. The implementation
+   of every field in each service discovery mechanism is left to the implementation. The expectation is not for all of
+   them to be implemented.
+2. Once released, refactor the configuration generation logic to reuse `ScrapeConfig`. In parallel, add other service
+   discovery mechanisms to the CRD and complete the implementation of `file_sd_configs`, `static_configs` and
+   `http_sd_configs`.