OpenCensus to OpenTelemetry metric exporter

[zoercai]

The OpenTelemetry merger announcement promised straightforward backwards compatibility with OpenCensus.

This PR adds an OpenCensus bridge for metrics, which allows applications and libraries that are instrumented with OpenTelemetry, but depend on other libraries instrumented with OpenCensus, to export metrics from both OpenTelemetry and OpenCensus.

The bridge maps all OpenCensus metric data to OpenTelemetry metric data, which are then exported by any configured OpenTelemetry exporter.

For OpenCensus spans to be exported as OpenTelemetry spans, this bridge needs to be added as a dependency. An extra line of configuration is also necessary in addition to configuring the OpenTelemetry exporter. For example

// Normal OpenTelemetry exporter configuration
LoggingMetricExporter loggingMetricExporter = new LoggingMetricExporter();
// Extra line, creating the bridge exporter, with the OpenTelemetry exporter
OpenTelemetryMetricsExporter exporter = OpenTelemetryMetricsExporter.createAndRegister(loggingMetricExporter);

cc @nilebox @james-bebbington

[anuraaga]

Just curious is there anything special about metrics that let's us handle it at the export layer instead of instrumentation layer like we did for traces?

[zoercai]

@anuraaga mainly that metrics don't have parent-child relationships, which was the main reason for not handling traces at the export layer. Label propagation hasn't been implemented in OpenTelemetry either (https:/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/api.md#baggage-into-metric-labels) so it seems that it would be fine (and much easier) to export the metric data output rather than modifying the context.

[anuraaga]

Thanks

[anuraaga]

LGTM

[jkwatson]

Just one small refactoring needed; otherwise looks good to go.

[jkwatson]

Thanks!