Avoid existing data (if present) in the received json event from being overwritten by added ECS metadata.

[kxs-who]

According to the documentation (https://www.elastic.co/guide/en/logstash/current/plugins-inputs-http.html#plugins-inputs-http-ecs_metadata), the following ECS metadata is populated by the http input plugin:

• Host IP address
• Complete HTTP headers
• HTTP version
• client user agent
• host domain and port
• HTTP method
• Query path
• Request content length
• Request mime type

The following issue was logged because the host field was being overwritten: #61
We would like the http input plugin to have a way to prevent overwriting not just the host metadata but all metadata.