datadog-apm: deprecate opentelemetry datadog exporter (#2517)

Author: mabdinur

docs/examples/datadog_exporter/README.rst

@@ -1,5 +1,8 @@
-Datadog Exporter
-================
+Datadog Span Exporter
+=====================
+
+.. warning:: This exporter has been deprecated. To export your OTLP traces from OpenTelemetry SDK directly to Datadog Agent, please refer to `OTLP Ingest in Datadog Agent <https://docs.datadoghq.com/tracing/setup_overview/open_standards/#otlp-ingest-in-datadog-agent>`_ .
+
 
 These examples show how to use OpenTelemetry to send tracing data to Datadog.