[Backport 7.x] Add ability to disable the http agent (#1258)

Co-authored-by: Tomas Della Vedova <delvedor@users.noreply.github.com>

docs/configuration.asciidoc

@@ -119,7 +119,8 @@ _Default:_ `null`
 
 |`agent`
 a|`http.AgentOptions, function` - http agent https://nodejs.org/api/http.html#http_new_agent_options[options], 
-or a function that returns an actual http agent instance. +
+or a function that returns an actual http agent instance. If you want to disable the http agent use entirely
+(and disable the `keep-alive` feature), set the agent to `false`. +
 _Default:_ `null`
 [source,js]
 ----
@@ -132,6 +133,12 @@ const client = new Client({
   node: 'http://localhost:9200',
   agent: () => new CustomAgent()
 })
+
+const client = new Client({
+  node: 'http://localhost:9200',
+  // Disable agent and keep-alive
+  agent: false
+})
 ----
 
 |`nodeFilter`