Prometheus: Generate command line flags dynamically

This proposes a means for generating the command line flags for
configuring the Prometheus service via the values file instead of
templating out the command line flags used for the service. This
allows flexibility in choosing which flags and values to use when
deploying Prometheus, without needing to modify the chart itself

Change-Id: I74845b96e213403ad743724137a82ce2c78fcd1f

prometheus/templates/_helpers.tpl

@@ -0,0 +1,48 @@
+{{/*
+Copyright 2017 The Openstack-Helm Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/}}
+
+# This function generates the command line flags passed to Prometheus at time of
+# execution. This allows the Prometheus service configuration to be flexible, as
+# the only way to define Prometheus's configuration is via command line flags.
+# The yaml definition for these flags uses the full yaml path as the key, and
+# replaces underscores with hyphens to match the syntax required for the flags
+# generated (This is required due to Go's yaml parsing capabilities).
+# For example:
+#
+# conf:
+#   prometheus:
+#     command_line_flags:
+#       storage.tsdb.max_block_duration: 2h
+#
+# Will generate the following flag:
+#   --storage.tsdb.max-block-duration=2h
+#
+# Prometheus's command flags can be found by either running 'prometheus -h' or
+# 'prometheus --help-man'
+
+{{- define "prometheus.utils.command_line_flags" -}}
+{{- range $flag, $value := . }}
+{{- $flag := $flag | replace "_" "-" -}}
+{{- if eq $flag "web.enable-admin-api" -}}
+{{- if $value -}}
+{{- printf "--%s" $flag }}
+{{- end -}}
+{{- else -}}
+{{- $value := $value | toString }}
+{{- printf "--%s=%s " $flag $value }}
+{{- end -}}
+{{- end -}}
+{{- end -}}

prometheus/templates/bin/_prometheus.sh.tpl

@@ -20,18 +20,8 @@ set -ex
 COMMAND="${@:-start}"
 
 function start () {
-  exec /bin/prometheus \
-    --config.file=/etc/config/prometheus.yml \
-    --log.level={{ .Values.conf.prometheus.log.level | quote }} \
-    --query.max-concurrency={{ .Values.conf.prometheus.query.max_concurrency }} \
-    --storage.tsdb.path={{ .Values.conf.prometheus.storage.tsdb.path }} \
-    --storage.tsdb.retention={{ .Values.conf.prometheus.storage.tsdb.retention }} \
-    --storage.tsdb.min-block-duration={{ .Values.conf.prometheus.storage.tsdb.min_block_duration }} \
-    --storage.tsdb.max-block-duration={{ .Values.conf.prometheus.storage.tsdb.max_block_duration }} \
-    {{ if .Values.conf.prometheus.web_admin_api.enabled }}
-    --web.enable-admin-api \
-    {{ end }}
-    --query.timeout={{ .Values.conf.prometheus.query.timeout }}
+{{ $flags := include "prometheus.utils.command_line_flags" .Values.conf.prometheus.command_line_flags }}
+  exec /bin/prometheus --config.file=/etc/config/prometheus.yml {{ $flags }}
 }
 
 function stop () {

prometheus/values.yaml

@@ -182,20 +182,17 @@ manifests:
 
 conf:
   prometheus:
-    storage:
-      tsdb:
-        path: /var/lib/prometheus/data
-        retention: 7d
-        min_block_duration: 2h
-        max_block_duration: 6h
-    log:
-      format: logger:stdout?json=true
-      level: info
-    query:
-      max_concurrency: 20
-      timeout: 2m
-    web_admin_api:
-      enabled: false
+    # Consumed by a prometheus helper function to generate the command line flags
+    # for configuring the prometheus service
+    command_line_flags:
+      log.level: info
+      query.max_concurrency: 20
+      query.timeout: 2m
+      storage.tsdb.path: /var/lib/prometheus/data
+      storage.tsdb.retention: 7d
+      storage.tsdb.min_block_duration: 2h
+      storage.tsdb.max_block_duration: 2h
+      web.enable_admin_api: false
     scrape_configs:
       global:
         scrape_interval: 60s