Add topic scope to report_metric

[YadhuPrakash]

Summary:

Librdkafka emits topic level stats like so:

{
  "name": "rdkafka#producer-1",
  "client_id": "rdkafka",
  "ts": 5016483227792,
  "brokers": {},
  "topics": {
    "test": {
      "topic": "test",
      "batchsize": {},
      "batchcnt": {},
      "partitions": {}
      }
    }
  },
}

This update allows users to subscribe to topic level stats as a metric with: RdKafkaMetric.new(:gauge, :topics, 'topics.batchcnt.avg', %w[batchcnt avg])

Related issue: https://github.com/karafka/waterdrop/issues/495

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