Add Makefile and README

prometheus/apache-http-mixin/Makefile

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+JSONNET_FMT := jsonnetfmt -n 2 --max-blank-lines 1 --string-style s --comment-style s
+
+.PHONY: all
+all: build dashboards_out prometheus_alerts.yaml
+
+vendor: jsonnetfile.json
+	jb install
+
+.PHONY: build
+build: vendor
+
+.PHONY: fmt
+fmt:
+	find . -name 'vendor' -prune -o -name '*.libsonnet' -print -o -name '*.jsonnet' -print | \
+		xargs -n 1 -- $(JSONNET_FMT) -i
+
+.PHONY: lint
+lint: build
+	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
+
+dashboards_out: mixin.libsonnet config.libsonnet $(wildcard dashboards/*)
+	@mkdir -p dashboards_out
+	mixtool generate dashboards mixin.libsonnet -d dashboards_out
+
+prometheus_alerts.yaml: mixin.libsonnet alerts/*.libsonnet
+	mixtool generate alerts mixin.libsonnet -a prometheus_alerts.yaml
+
+.PHONY: clean
+clean:
+	rm -rf dashboards_out prometheus_alerts.yaml

prometheus/apache-http-mixin/README.md

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+# Apache HTTP server mixin
+
+Apache HTTP mixin is a set of configurable Grafana dashboards and alerts based on the metrics exported by the [Apache exporter](https://github.com/Lusitaniae/apache_exporter).
+
+![image](https://user-images.githubusercontent.com/14870891/167886371-92c60942-bbfa-43d2-ba16-ee13e629020a.png)
+
+Install tools before generating dashboards and alerts:
+
+```bash
+go install github.com/jsonnet-bundler/jsonnet-bundler/cmd/jb@latest
+go install github.com/monitoring-mixins/mixtool/cmd/mixtool@latest
+```
+
+For linting and formatting, you would also need `mixtool` and `jsonnetfmt` installed. If you
+have a working Go development environment, it's easiest to run the following:
+
+```bash
+go install github.com/google/go-jsonnet/cmd/jsonnetfmt@latest
+```
+
+The files in `dashboards_out` need to be imported
+into your Grafana server.  The exact details will be depending on your environment.
+
+`prometheus_alerts.yaml` needs to be imported into Prometheus.
+
+Edit `config.libsonnet` if required and then build JSON dashboard files for Grafana:
+
+```bash
+make
+```
+
+For more advanced uses of mixins, see
+https://github.com/monitoring-mixins/docs.