code: function setTag(key, value, obj) {
    if (!obj || !key) return
    var skey = removeInvalidChars(key)
    if (value) {
      value = String(value)
    }
    obj[skey] = value
    return obj
  }
docs: Convert values of the tag to be string to be compatible
  with the apm server prior to <6.7 version
  
  TODO: Remove string conversion in the next major release since
  support for boolean and number in the APM server has landed in 6.7
  https://github.com/elastic/apm-server/pull/1712/