logstash datadog metrics output plugins can not resolve variable in dd_tags

[jinesh90]

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For all general issues, please provide the following details for fast resolution:

• Version: logstash-output-datadog_metrics (3.0.4)
• Operating System: ubuntu 18.06 GA
• Config File (if you have sensitive info, please remove it):
  {code}
  input {
  tcp {
  codec => fluent
  port => 4400
  }
  }
  filter{
  json {
  source => "log"

}
}
output {
datadog_metrics {
api_key => <api_key>
metric_name => "test.metrics"
metric_type => "counter"
metric_value => 1
dd_tags => ["servicename:%{service}"]
}
}
{code}

• Sample Data:
  {code}

{code}
{
"traceId" => "1234",
"service" => "testlog",
"@Version" => "1",
"log" => "{"timeMillis":1563999134904,"thread":"main","level":"INFO","loggerName":"logger","message":"Starting registrar","endOfBatch":false,"loggerFqcn":"org.apache.logging.slf4j.Log4jLogger","threadId":1,"threadPriority":5,"timestamp":"2019-07-24T20:12:14.904","build":"11112","traceId":"1234","service":"testlog"}\r",
"container_name" => "container1",
"timestamp" => "2019-07-24T20:12:14.904",
"host" => "12.123.12.12",
"@timestamp" => 2019-07-24T20:12:14.000Z,
"thread" => "main",
"tags" => [
[0] "f8cdd0aeb906",
[1] "servicename:testlog"
],
"region" => "us-west-2",
"source" => "stdout",
"service_name" => "testlog",
"message" => "Starting registrar",
"build" => "11112",
"timeMillis" => 1563999134904,
"loggerName" => "logger",
"container_id" => "f65421a",
"level" => "INFO",
"endOfBatch" => false,
"port" => 41704
}
{code}

• Steps to Reproduce:

1. use the logstash.conf according to config.
2. make sure you have installed logstash-output-datadog_metrics (3.0.4)
3. in logstash config, output part, make sure user filled correct information about datadog key and other params
4. use json parser as filter. and fluentd as input mentioned in logstash.conf.
5. in dd_tag field, use any of parsed json value,( in this case, "%{service}")

Issue:
in data dog, metrics created as per expectation, but tag is not resolving value of "%{service}", it should be "testlog" but it takes as it is.