PushHttpMetricsReporter should not convert metric value to double

Currently PushHttpMetricsReporter will convert value from KafkaMetric.metricValue() to double. This will not work for non-numerical metrics such as version in AppInfoParser whose value can be string. This has caused issue for PushHttpMetricsReporter which in turn caused system test kafkatest.tests.client.quota_test.QuotaTest.test_quota to fail with the following exception:  

 File "/opt/kafka-dev/tests/kafkatest/tests/client/quota_test.py", line 196, in validate     metric.value for k, metrics in producer.metrics(group='producer-metrics', name='outgoing-byte-rate', client_id=producer.client_id) for metric in metrics ValueError: max() arg is an empty sequence

Since we allow metric value to be object, PushHttpMetricsReporter should also read metric value as object and pass it to the http server.