pipeline: input: http: add a description about tag_key

[nokute78]

This patch is to add missing tag_key option. https://github.com/fluent/fluent-bit/blob/v1.9.6/plugins/in_http/http.c#L146

Key	Description	default
tag_key	Specify the key name to overwrite a tag. If set, the tag will be overwritten by a value of the key.