Metrics SDK: Add Metrics exemplar

[lalitb]

Specs: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar

• A metrics exemplar is a trace that is representative of the metrics being collected. The spec requires that exemplars be implemented in a way such that they may be disabled and that if disabled they should have no performance impact (collection should be disabled, not just export). When enabled, the exemplars will come from the currently active context.
  Properties: Value, Time, Attributes, Trace ID, Span ID

• ExemplarFilter determines if a measurement should be sampled
  Properties - value of measurement, attributes of measurement, context of measurement, timestamp

• ExemplarReservoir provides a method to offer measurements to the reservoir and to collect accumulated exemplars

[github-actions]

This issue was marked as stale due to lack of activity. It will be closed in 7 days if no furthur activity occurs.

[lalitb]

The record API / SDK needs to be modified too to pass span context, which would be required to get trace id and span id.

template <class T>
class UpDownCounter : public SynchronousInstrument
{
  virtual void Add(T value, opentelemetry::context::Context context) noexcept = 0;
  virtual void Add(T value, const common::KeyValueIterable &attributes, opentelemetry::context::Context context) noexcept = 0;
}