update add serviceName keyword default field to exemplar fields

[YANG-DB]

Changes

update data-prepper's metrics index template with serviceName field under exemplar section:

      "exemplar": {
          "properties": {
            "time": {
              "type": "date"
            },
            "traceId": {
              "ignore_above": 256,
              "type": "keyword"
            },
            "serviceName": {
              "ignore_above": 256,
              "type": "keyword"
            },
            "spanId": {
              "ignore_above": 256,
              "type": "keyword"
            }
          }
        } 

related issues

#147

additional context

exemplar proto