Expose cache setting in UserAgentPlugin (#46533)

The setting was not registered. Also documentation has been added.

docs/reference/ingest/processors/user-agent.asciidoc

@@ -85,3 +85,14 @@ In practice, it will make most sense for any custom regex file to be a variant o
 or a customised version.
 
 The default file included in `ingest-user-agent` is the `regexes.yaml` from uap-core: https://github.com/ua-parser/uap-core/blob/master/regexes.yaml
+
+[[ingest-user-agent-settings]]
+===== Node Settings
+
+The `user_agent` processor supports the following setting:
+
+`ingest.user_agent.cache_size`::
+
+    The maximum number of results that should be cached. Defaults to `1000`.
+
+Note that these settings are node settings and apply to all `user_agent` processors, i.e. there is one cache for all defined `user_agent` processors.

modules/ingest-user-agent/src/main/java/org/elasticsearch/ingest/useragent/IngestUserAgentPlugin.java

@@ -32,6 +32,7 @@ import java.nio.file.PathMatcher;
 import java.nio.file.StandardOpenOption;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.stream.Stream;
 
@@ -86,4 +87,8 @@ public class IngestUserAgentPlugin extends Plugin implements IngestPlugin {
         return Collections.unmodifiableMap(userAgentParsers);
     }
 
+    @Override
+    public List<Setting<?>> getSettings() {
+        return Collections.singletonList(CACHE_SIZE_SETTING);
+    }
 }