Add link to exemplars to specification/metrics/README.md (#3764)

Co-authored-by: Armin Ruech <[email protected]>

specification/metrics/README.md

@@ -30,7 +30,7 @@ Given there are many well-established metrics solutions that exist today, it is
 important to understand the goals of OpenTelemetry’s metrics effort:
 
 * **Being able to connect metrics to other signals**. For example, metrics and
- traces can be correlated via exemplars, and metrics attributes can be enriched
+ traces can be correlated via [exemplars](data-model.md#exemplars), and metrics attributes can be enriched
  via [Baggage](../baggage/api.md) and [Context](../context/README.md).
  Additionally, [Resource](../resource/sdk.md) can be applied to
  [logs](../overview.md#log-signal)/[metrics](../overview.md#metric-signal)/[traces](../overview.md#tracing-signal)