Support metric exporter

[ymotongpoo]

Replace #14 with this PR.

• basic implementation of metric exporter
• add example

[rghetia]

If not tested with SD backend, please test one metric of type Observer and Counter with the backend.
See other inline comments..

[rghetia]

@nilebox can you please review this as well.

[ymotongpoo]

The sample sometimes show the rpc error as follows that is relevant to #25 :

2020/05/16 00:29:13 Submitted data: counter 118, observer 12.49
2020/05/16 00:29:14 Submitted data: counter 181, observer 12.59
2020/05/16 00:29:15 Submitted data: counter 191, observer 12.54
2020/05/16 00:29:16 Submitted data: counter 175, observer 12.34
Metrics Accumulator error: rpc error: code = InvalidArgument desc = Field timeSeries[1].points[0].interval.start_time had an invalid value of "2020-05-15T08:29:16-07:00": The start time must be equal to the end time (2020-05-15T08:29:17-07:00) for the gauge metric 'custom.googleapis.com/opentelemetry/observer-a'.
2020/05/16 00:29:17 Submitted data: counter 161, observer 12.74
2020/05/16 00:29:18 Submitted data: counter 167, observer 12.74
2020/05/16 00:29:19 Submitted data: counter 119, observer 12.54

[rghetia]

The sample sometimes show the rpc error as follows that is relevant to #25 :

2020/05/16 00:29:13 Submitted data: counter 118, observer 12.49
2020/05/16 00:29:14 Submitted data: counter 181, observer 12.59
2020/05/16 00:29:15 Submitted data: counter 191, observer 12.54
2020/05/16 00:29:16 Submitted data: counter 175, observer 12.34
Metrics Accumulator error: rpc error: code = InvalidArgument desc = Field timeSeries[1].points[0].interval.start_time had an invalid value of "2020-05-15T08:29:16-07:00": The start time must be equal to the end time (2020-05-15T08:29:17-07:00) for the gauge metric 'custom.googleapis.com/opentelemetry/observer-a'.
2020/05/16 00:29:17 Submitted data: counter 161, observer 12.74
2020/05/16 00:29:18 Submitted data: counter 167, observer 12.74
2020/05/16 00:29:19 Submitted data: counter 119, observer 12.54

you could not set the start_time for gauge metric. then it probably won't complain.

[ymotongpoo]

Example: Counter is working. Observer fails with the following error message.

2020/05/16 11:20:41 Submitted data: counter 141, observer 12.79
2020/05/16 11:20:51 Submitted data: counter 119, observer 12.64
2020/05/16 11:21:01 Submitted data: counter 173, observer 12.34
Metrics Accumulator error: rpc error: code = InvalidArgument desc = One or more TimeSeries could not be written: Value type for metric custom.googleapis.com/opentelemetry/observer-a conflicts with the existing value type (DOUBLE).: timeSeries[1]
2020/05/16 11:21:11 Submitted data: counter 100, observer 12.49
2020/05/16 11:21:21 Submitted data: counter 151, observer 12.69```