feat(mixin): Add grafana mixins support for postgres exporter

postgres_mixin/.gitignore

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+/alerts.yaml
+/rules.yaml
+dashboards_out

postgres_mixin/Makefile

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+JSONNET_FMT := jsonnetfmt -n 2 --max-blank-lines 2 --string-style s --comment-style s
+
+default: build
+
+all: fmt lint build clean
+
+fmt:
+	find . -name 'vendor' -prune -o -name '*.libsonnet' -print -o -name '*.jsonnet' -print | \
+		xargs -n 1 -- $(JSONNET_FMT) -i
+
+lint:
+	find . -name 'vendor' -prune -o -name '*.libsonnet' -print -o -name '*.jsonnet' -print | \
+		while read f; do \
+			$(JSONNET_FMT) "$$f" | diff -u "$$f" -; \
+		done
+
+	mixtool lint mixin.libsonnet
+
+build:
+	mixtool generate all mixin.libsonnet
+
+clean:
+	rm -rf dashboards_out alerts.yaml rules.yaml

postgres_mixin/README.md

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+# Postgres Mixin
+
+_This is a work in progress. We aim for it to become a good role model for alerts
+and dashboards eventually, but it is not quite there yet._
+
+The Postgres Mixin is a set of configurable, reusable, and extensible alerts and
+dashboards based on the metrics exported by the Postgres Exporter. The mixin creates
+recording and alerting rules for Prometheus and suitable dashboard descriptions
+for Grafana.
+
+To use them, you need to have `mixtool` and `jsonnetfmt` installed. If you
+have a working Go development environment, it's easiest to run the following:
+```bash
+$ go get github.com/monitoring-mixins/mixtool/cmd/mixtool
+$ go get github.com/google/go-jsonnet/cmd/jsonnetfmt
+```
+
+You can then build the Prometheus rules files `alerts.yaml` and
+`rules.yaml` and a directory `dashboard_out` with the JSON dashboard files
+for Grafana:
+```bash
+$ make build
+```
+
+For more advanced uses of mixins, see
+https://github.com/monitoring-mixins/docs.