feat(ext): Add Heartbeat extension

Closes #672

.github/workflows/scripts/run-tests-heartbeat.sh

@@ -0,0 +1,61 @@
+#!/usr/bin/env bash
+
+set -eu
+set -o pipefail
+
+
+source "$(dirname ${BASH_SOURCE[0]})/lib/testing.sh"
+
+
+cid_es="$(container_id elasticsearch)"
+cid_mb="$(container_id heartbeat)"
+
+ip_es="$(service_ip elasticsearch)"
+ip_mb="$(service_ip heartbeat)"
+
+log 'Waiting for readiness of Elasticsearch'
+poll_ready "$cid_es" "http://${ip_es}:9200/" -u 'elastic:testpasswd'
+
+log 'Waiting for readiness of Heartbeat'
+poll_ready "$cid_mb" "http://${ip_mb}:5066/?pretty"
+
+# We expect to find heartbeat entries for the 'elasticsearch' HTTP service
+# using the following query:
+#
+#   agent.type:"heartbeat"
+#   AND monitor.type:"http"
+#   AND url.domain:"elasticsearch"
+#
+log 'Searching a document generated by Heartbeat'
+
+declare response
+declare -i count
+
+declare -i was_retried=0
+
+# retry for max 60s (30*2s)
+for _ in $(seq 1 30); do
+	response="$(curl "http://${ip_es}:9200/heartbeat-*/_search?q=agent.type:%22heartbeat%22%20AND%20monitor.type:%22http%22%20AND%20url.domain:%22elasticsearch%22&pretty" -s -u elastic:testpasswd)"
+
+	set +u  # prevent "unbound variable" if assigned value is not an integer
+	count="$(jq -rn --argjson data "${response}" '$data.hits.total.value')"
+	set -u
+
+	if (( count > 0 )); then
+		break
+	fi
+
+	was_retried=1
+	echo -n 'x' >&2
+	sleep 2
+done
+if ((was_retried)); then
+	# flush stderr, important in non-interactive environments (CI)
+	echo >&2
+fi
+
+echo "$response"
+if (( count == 0 )); then
+	echo 'Expected at least 1 document'
+	exit 1
+fi