How to: retrieve trace id for exemplar
span = trace.get_current_span()
trace_id = trace.format_trace_id(
      span.get_span_context().trace_id)

REQUESTS_PROCESSING_TIME.labels(method=method, path=path, app_name=self.app_name).observe(
      after_time - before_time, exemplar={'TraceID': trace_id}
)
```

Because exemplars is a new datatype proposed in [OpenMetrics](https://github.com/prometheus/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars), `/metrics` have to use `CONTENT_TYPE_LATEST` and `generate_latest` from `prometheus_client.openmetrics.exposition` module instead of `prometheus_client` module ([Prometheus Client Document](https://prometheus.github.io/client_python/instrumenting/exemplars/)). Otherwise using the wrong generate_latest the exemplars dict behind Counter and Histogram will never show up, and using the wrong CONTENT_TYPE_LATEST will cause Prometheus scraping to fail.

```py