Migrate HTTP SD docs from docs repo

See discussion in https://github.com/prometheus/docs/pull/1975



Signed-off-by: Julius Volz <julius.volz@gmail.com>

docs/disabled_features.md

 ---
 title: Disabled Features
-sort_rank: 10
+sort_rank: 11
 ---
 
 # Disabled Features
@@ -52,4 +52,4 @@ The remote write receiver allows Prometheus to accept remote write requests from
 
 [OpenMetrics](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars) introduces the ability for scrape targets to add exemplars to certain metrics. Exemplars are references to data outside of the MetricSet. A common use case are IDs of program traces.
 
-Exemplar storage is implemented as a fixed size circular buffer that stores exemplars in memory for all series. Enabling this feature will enable the storage of exemplars scraped by Prometheus. The flag `storage.exemplars.exemplars-limit` can be used to control the size of circular buffer by # of exemplars. An exemplar with just a `traceID=<jaeger-trace-id>` uses roughly 100 bytes of memory via the in-memory exemplar storage. If the exemplar storage is enabled, we will also append the exemplars to WAL for local persistence (for WAL duration).
\ No newline at end of file
+Exemplar storage is implemented as a fixed size circular buffer that stores exemplars in memory for all series. Enabling this feature will enable the storage of exemplars scraped by Prometheus. The flag `storage.exemplars.exemplars-limit` can be used to control the size of circular buffer by # of exemplars. An exemplar with just a `traceID=<jaeger-trace-id>` uses roughly 100 bytes of memory via the in-memory exemplar storage. If the exemplar storage is enabled, we will also append the exemplars to WAL for local persistence (for WAL duration).

docs/http_sd.md

+---
+title: HTTP SD
+sort_rank: 7
+---
+
+# Writing HTTP Service Discovery
+
+Prometheus provides a generic [HTTP Service Discovery](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#http_sd_config),
+that enables it to discover targets over an HTTP endpoint.
+
+The HTTP Service Discovery is complimentary to the supported service
+discovery mechanisms, and is an alternative to [File-based Service Discovery](https://prometheus.io/docs/guides/file-sd/#use-file-based-service-discovery-to-discover-scrape-targets).
+
+## Comparison between File-Based SD and HTTP SD
+
+Here is a table comparing our two generic Service Discovery implementations.
+
+| Item | File SD | HTTP SD |
+| ---- | ------- | ------- |
+| Event Based | Yes, via inotify | No |
+| Update frequency | Instant, thanks to inotify | Following refresh_interval |
+| Format | Yaml or JSON | JSON |
+| Transport | Local file | HTTP/HTTPS |
+| Security | File-Based security | TLS, Basic auth, Authorization header, OAuth2 |
+
+## Requirements of HTTP SD endpoints
+
+If you implement an HTTP SD endpoint, here is a few requirements you should be
+aware of.
+
+The response is consumed as is, unmodified. On each refresh interval (default: 1
+minute), Prometheus will perform a GET request to the HTTP SD endpoint. The GET
+request contains a `X-Prometheus-Refresh-Interval-Seconds` HTTP header with the
+refresh interval.
+
+The SD endpoint must answer with an HTTP 200 response, with the HTTP Header
+`Content-Type: application/json`. The answer must be UTF-8 formatted.
+If no targets should be transmitted, HTTP 200 must also be emitted, with
+an empty list `[]`. Target lists are unordered.
+
+Prometheus caches target lists. If an error occurs while fetching an updated
+targets list, Prometheus keeps using the current targets list. The targets list
+is not saved across restart.
+
+The whole list of targets must be returned on every scrape. There is no support
+for incremental updates. A Prometheus instance does not send its hostname and it
+is not possible for a SD endpoint to know if the SD requests is the first one
+after a restart or not.
+
+The URL to the HTTP SD is not considered secret. The authentication, and any API
+keys should be passed with the appropriate authentication mechanisms. Prometheus
+supports TLS authentication, basic authentication, OAuth2, and authorization
+headers.
+
+## HTTP_SD format
+
+```json
+[
+  {
+    "targets": [ "<host>", ... ],
+    "labels": {
+      "<labelname>": "<labelvalue>", ...
+    }
+  },
+  ...
+]
+```
+
+
+Examples:
+
+```json
+[
+    {
+        "targets": ["10.0.10.2:9100", "10.0.10.3:9100", "10.0.10.4:9100", "10.0.10.5:9100"],
+        "labels": {
+            "__meta_datacenter": "london",
+            "__meta_prometheus_job": "node"
+        }
+    },
+    {
+        "targets": ["10.0.40.2:9100", "10.0.40.3:9100"],
+        "labels": {
+            "__meta_datacenter": "london",
+            "__meta_prometheus_job": "alertmanager"
+        }
+    },
+    {
+        "targets": ["10.0.40.2:9093", "10.0.40.3:9093"],
+        "labels": {
+            "__meta_datacenter": "newyork",
+            "__meta_prometheus_job": "alertmanager"
+        }
+    }
+]
+```

docs/management_api.md

 ---
 title: Management API
-sort_rank: 7
+sort_rank: 8
 ---
 
 # Management API

docs/migration.md

 ---
 title: Migration
-sort_rank: 8
+sort_rank: 9
 ---
 
 # Prometheus 2.0 migration guide

docs/stability.md

 ---
 title: API Stability
-sort_rank: 9
+sort_rank: 10
 ---
 
 # API Stability Guarantees