Update docs/configuring-playbook-prometheus-postgres.md: add the copyright header before deleting the file

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-prometheus-postgres.md

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+SPDX-FileCopyrightText: 2021 Raymond Coetzee
+SPDX-FileCopyrightText: 2021 WobbelTheBear
+SPDX-FileCopyrightText: 2022 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2024 Suguru Hirahara
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 # Enabling metrics and graphs for Postgres (optional)
 
 Expanding on the metrics exposed by the [synapse exporter and the node exporter](configuring-playbook-prometheus-grafana.md), the playbook enables the [postgres exporter](https://github.com/prometheus-community/postgres_exporter) that exposes more detailed information about what's happening on your postgres database.