docs: add HTTP API request examples

Closes #1847

docs/http_websocket_support.md

@@ -4,6 +4,16 @@ If enabled in the config, *rtpengine* can handle requests made to it via HTTP,
 HTTPS, or WebSocket (WS or WSS) connections. The supported HTTP URIs and
 WebSocket subprotocols are described below.
 
+Enable the HTTP listener with `--listen-http=[IP|HOSTNAME:]PORT` (or the
+equivalent `listen-http` setting in the config file), for example:
+
+    --listen-http=127.0.0.1:2225
+
+The examples below assume *rtpengine* is listening on `http://127.0.0.1:2225`.
+Command dictionaries follow the *ng* control protocol; see
+[The NG Control Protocol](ng_control_protocol.md) for the full command
+reference.
+
 ## Dummy Test Interfaces
 
 For HTTP and HTTPS, the URI `/ping` is provided, which simply responds with
@@ -11,6 +21,11 @@ For HTTP and HTTPS, the URI `/ping` is provided, which simply responds with
 `echo.rtpengine.com` is provided, which simply echoes back any messages that
 are sent to it.
 
+Example:
+
+    curl -s http://127.0.0.1:2225/ping
+    # -> pong
+
 ## CLI Interface
 
 This interface supports the same commands as the CLI tool `rtpengine-ctl` that
@@ -24,6 +39,14 @@ and produces one message in response. The format of each response is exactly
 the same as produced by the CLI tool `rtpengine-ctl` and therefore meant for
 plain text representation.
 
+Examples:
+
+    curl -s 'http://127.0.0.1:2225/cli/list+totals'
+
+    curl -s -X POST -H 'Content-Type: text/plain' \
+        --data 'list totals' \
+        http://127.0.0.1:2225/cli
+
 ## *ng* Protocol Interface
 
 This interface can be used to send and receive *ng* protocol messages over HTTP
@@ -48,6 +71,60 @@ unique cookie. In other words, each payload is just a plain *bencode*
 dictionary or a *JSON* object. Therefore the content-type `application/json`
 can also be used for HTTP `POST`.
 
+### Examples using `/ng-plain`
+
+The `/ng-plain` endpoint is the simplest way to send *ng* commands over HTTP.
+Send a JSON object as the request body:
+
+    curl -s -X POST http://127.0.0.1:2225/ng-plain \
+        -H 'Content-Type: application/json' \
+        -d '{"command":"ping"}'
+    # -> {"result":"pong"}
+
+`offer`, `answer`, and `delete` use the same request shape. A minimal `offer`:
+
+    curl -s -X POST http://127.0.0.1:2225/ng-plain \
+        -H 'Content-Type: application/json' \
+        -d '{"command":"offer","call-id":"test-call-1","from-tag":"from-tag-1","sdp":"v=0\r\no=- 1 1 IN IP4 198.51.100.1\r\ns=test\r\nc=IN IP4 198.51.100.1\r\nt=0 0\r\nm=audio 10000 RTP/AVP 0\r\na=rtpmap:0 PCMU/8000\r\n"}'
+
+A matching `answer` (same `call-id` and `from-tag`, plus a `to-tag`):
+
+    curl -s -X POST http://127.0.0.1:2225/ng-plain \
+        -H 'Content-Type: application/json' \
+        -d '{"command":"answer","call-id":"test-call-1","from-tag":"from-tag-1","to-tag":"to-tag-1","sdp":"v=0\r\no=- 2 2 IN IP4 198.51.100.2\r\ns=test\r\nc=IN IP4 198.51.100.2\r\nt=0 0\r\nm=audio 10002 RTP/AVP 0\r\na=rtpmap:0 PCMU/8000\r\n"}'
+
+Tear the call down with `delete`:
+
+    curl -s -X POST http://127.0.0.1:2225/ng-plain \
+        -H 'Content-Type: application/json' \
+        -d '{"command":"delete","call-id":"test-call-1","from-tag":"from-tag-1","to-tag":"to-tag-1"}'
+
+### Examples using `/ng`
+
+The `/ng` endpoint expects the classic *ng* framing: a unique cookie, a single
+space, then the bencode or JSON dictionary. The content-type must be
+`application/x-rtpengine-ng`. The cookie must be unique for each distinct
+request; reusing a cookie is treated as a retransmission of the earlier
+request.
+
+Example with JSON:
+
+    curl -s -X POST http://127.0.0.1:2225/ng \
+        -H 'Content-Type: application/x-rtpengine-ng' \
+        --data-binary '5323_1 {"command":"ping"}'
+    # -> 5323_1 {"result":"pong"}
+
+The same framing with bencode:
+
+    curl -s -X POST http://127.0.0.1:2225/ng \
+        -H 'Content-Type: application/x-rtpengine-ng' \
+        --data-binary '5323_1 d7:command4:pinge'
+    # -> 5323_1 d6:result4:ponge
+
 ## Prometheus Stats Exporter
 
 The Prometheus metrics can be found under the URI `/metrics`.
+
+Example:
+
+    curl -s http://127.0.0.1:2225/metrics