Update README.md

Author: taonic

cloud/observability/promql-to-dd-go/README.md

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-PromQL to Datadog Go
+⚠️ We recommend using Datadog's official Temporal Cloud integration: https://docs.datadoghq.com/integrations/temporal_cloud/ over this stop-gap legacy solution.
+
+PromQL to Datadog Go (Legacy)
 =================
 
 The goal of this Golang implementation is to demonstrate the minimum work necessary to read recently generated metrics from a Temporal Cloud account using the Prometheus API and import them into DataDog while handling some common edge and error cases.