test: Add a Prometheus integration test

This patch adds a Prometheus integration test, that runs a Prometheus
instance which scrapes chasquid, and then does a basic query to ensure
the scraping is working correctly.

test/t-23-prometheus/run.sh

@@ -0,0 +1,53 @@
+#!/bin/bash
+
+set -e
+. "$(dirname "$0")/../util/lib.sh"
+
+init
+check_hostaliases
+
+# This is an integration test with Prometheus, so skip if it is not present.
+if ! prometheus --version > /dev/null; then
+	skip "prometheus not installed"
+fi
+if ! promtool help > /dev/null; then
+	skip "promtool not installed"
+fi
+
+# Start chasquid and prometheus.
+rm -rf .logs/ .prometheus/
+mkdir -p .logs .prometheus
+
+generate_certs_for testserver
+chasquid-util-user-add user@testserver secretpassword
+chasquid -v=2 --logfile=.logs/chasquid.log --config_dir=config &
+
+prometheus \
+	--config.file=prometheus.yml \
+	--storage.tsdb.path=.prometheus/ \
+	> .prometheus/log 2>&1 &
+
+
+# Wait until they're both up and running.
+wait_until_ready 1025
+wait_until "promtool check ready 2>/dev/null"
+
+# Send an email.
+smtpc user@testserver < content
+wait_for_file .mail/user@testserver
+mail_diff content .mail/user@testserver
+
+
+# Query Prometheus and validate that it has scraped chasquid correctly.
+function expect_value() {
+	promtool query instant http://localhost:9090 "$1" > ".logs/value-$1"
+	grep -q "=> $2" ".logs/value-$1"
+}
+
+# Note that it takes Prometheus ~5s to start scraping targets on the first
+# run, for reasons currently unknown.
+# If we don't clear up .prometheus/, then subsequent runs are faster, but
+# starting with an empty data directory makes the test more hermetic.
+wait_until expect_value chasquid_queue_putCount 1
+
+success