Add Google mtail for generating metrics from logs

This adds support for building an mtail image for generating metrics
from logs. These metrics can then be used to trigger alerts. For example,
one may want to track user logins, kernel errors, filesystem corruption
etc.

Change-Id: I0b3eee33c608b08f1e915a08096a1194d359805a
Partially-Implements: blueprint mtail

docker/prometheus/prometheus-mtail/Dockerfile.j2

@@ -0,0 +1,30 @@
+FROM {{ namespace }}/{{ image_prefix }}prometheus-base:{{ tag }}
+LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build_date }}"
+
+{% block prometheus_mtail_header %}{% endblock %}
+
+{% if base_arch not in ['x86_64'] %}
+
+RUN echo 'There are no mtail binaries for {{ base_arch }}' \
+    && /bin/false
+
+{% endif %}
+
+{% block prometheus_mtail_version %}
+ARG prometheus_mtail_version=v3.0.0-rc24
+ARG prometheus_mtail_url=https://github.com/google/mtail/releases/download/${prometheus_mtail_version}/mtail_${prometheus_mtail_version}_linux_amd64
+{% endblock %}
+
+{% block prometheus_mtail_install %}
+RUN curl -sSL -o /opt/mtail ${prometheus_mtail_url} \
+    && chmod +x /opt/mtail
+{% endblock %}
+
+{% block prometheus_mtail_footer %}{% endblock %}
+{% block footer %}{% endblock %}
+
+# NOTE(dszumski): We run as root so that mtail can scrape root owned logs
+# such as /var/log/secure which are mounted into the container as read only
+# for auditing purposes. Longer term we should use a more secure mechanism
+# for tailing these logs.
+USER root

releasenotes/notes/add-mtail-b806e87da3ae950d.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Add mtail, a utility for turning logs into metrics for use with
+    Prometheus.