Watcher history index has too many indexed fields

[markharwood]

A user reported an issue with the result.input.chain part of Watcher history documents being dynamic:true and accumulating large numbers of indexed fields over time. This caused failures relating to hitting indexed field limits.

Unless there's a particularly good business reason for indexing this data could we add this exclusion to the index template for watcher-history indices?

          "disabled_result_input_chain_fields" : {
            "path_match" : "result.input.chain",
            "mapping" : {
              "type" : "object",
              "enabled" : false
            }
          }

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)