update httpjson input docs

x-pack/filebeat/docs/inputs/input-httpjson.asciidoc

@@ -20,6 +20,7 @@ This input supports:
 * Pagination
 * Retries
 * Rate limiting
+* Proxying
 * Request transformations
 * Response transformations
 
@@ -383,6 +384,22 @@ This specifies SSL/TLS configuration. If the ssl section is missing, the host's
 CAs are used for HTTPS connections. See <<configuration-ssl>> for more
 information.
 
+[float]
+==== `request.proxy_url`
+
+This specifies proxy configuration in the form of `http[s]://<user>:<password>@<server name/ip>:<port>`
+
+["source","yaml",subs="attributes"]
+----
+filebeat.inputs:
+# Fetch your public IP every minute.
+- type: httpjson
+  config_version: 2
+  interval: 1m
+  request.url: https://api.ipify.org/?format=json
+  request.proxy_url: http://proxy.example:8080
+----
+
 [float]
 ==== `request.retry.max_attempts`