prometheus: simplify snapshot visualization

Make it *really easy* to look at `prometheus-snapshot.tgz` by putting a
shell script next to it that spins it up via docker, so you can just
run:

```
$ ./prometheus-docker-run.sh
[...]
ts=2022-02-09T09:07:06.254Z caller=main.go:896 level=info msg="Server is ready to receive web requests."
```

I got motivated to do this while working on the roachtest in cockroachdb#76147 but
I imagine it'll come in handy in general as well.

Release note: None

pkg/cmd/roachtest/prometheus/prometheus.go

@@ -16,6 +16,7 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"path/filepath"
 	"time"
 
 	"github.com/cockroachdb/cockroach/pkg/cmd/roachtest/option"
@@ -118,10 +119,9 @@ sudo systemd-run --unit prometheus --same-dir \
 	return &Prometheus{Config: cfg}, nil
 }
 
-// Snapshot takes a snapshot of prometheus and stores the snapshot in the given localPath
-func (pm *Prometheus) Snapshot(
-	ctx context.Context, c Cluster, l *logger.Logger, localPath string,
-) error {
+// Snapshot takes a snapshot of prometheus and stores the snapshot and a script to spin up
+// a docker instance for it to the given directory.
+func (pm *Prometheus) Snapshot(ctx context.Context, c Cluster, l *logger.Logger, dir string) error {
 	if err := c.RunE(
 		ctx,
 		pm.PrometheusNode,
@@ -130,12 +130,37 @@ func (pm *Prometheus) Snapshot(
 	); err != nil {
 		return err
 	}
+	if err := os.WriteFile(filepath.Join(dir, "prometheus-docker-run.sh"), []byte(`#!/bin/sh
+set -eu
+
+tar xf prometheus-snapshot.tar.gz
+snapdir=$(find data/snapshots -mindepth 1 -maxdepth 1 -type d)
+pyml=$(mktemp)
+chmod -R o+rw "${snapdir}" "${pyml}"
+
+cat <<EOF > "${pyml}"
+global:
+  scrape_interval: 10s
+  scrape_timeout: 5s
+EOF
+
+set -x
+# Worked as of v2.33.1 so you can hard-code that if necessary.
+docker run --privileged -p 9090:9090 \
+    -v "${pyml}:/etc/prometheus/prometheus.yml" -v "${PWD}/${snapdir}:/prometheus" \
+    prom/prometheus:latest \
+    --config.file=/etc/prometheus/prometheus.yml \
+    --storage.tsdb.path=/prometheus \
+    --web.enable-admin-api
+`), 0755); err != nil {
+		return err
+	}
 
 	return c.Get(
 		ctx,
 		l,
 		"/tmp/prometheus/prometheus-snapshot.tar.gz",
-		localPath,
+		dir,
 		pm.PrometheusNode,
 	)
 }

pkg/cmd/roachtest/tests/tpcc.go

@@ -1506,7 +1506,7 @@ func setupPrometheus(
 			snapshotCtx,
 			c,
 			t.L(),
-			filepath.Join(t.ArtifactsDir(), "prometheus-snapshot.tar.gz"),
+			t.ArtifactsDir(),
 		); err != nil {
 			t.L().Printf("failed to get prometheus snapshot: %v", err)
 		}