[docker] fix config bools, skip image stats option

Fixes #1342.
We should always cast boolean options from the config with
the `_is_affirmative` tool, otherwise you're exposed to
oddities like bool('false') == True.
This introduces a new option `collect_images_stats` that is
enabled by default and skips the collection of metrics
`docker.images.available` and `docker.images.intermediate` that
sometimes is very slow through docker API if you have a lot of
intermediate layer images.

checks.d/docker.py

@@ -12,6 +12,7 @@
 
 # project
 from checks import AgentCheck
+from config import _is_affirmative
 
 EVENT_TYPE = SOURCE_TYPE_NAME = 'docker'
 
@@ -127,7 +128,8 @@ def __init__(self, name, init_config, agentConfig):
 
     def check(self, instance):
         # Report image metrics
-        self._count_images(instance)
+        if _is_affirmative(instance.get('collect_images_stats', True)):
+            self._count_images(instance)
 
         # Get the list of containers and the index of their names
         containers, ids_to_names = self._get_and_count_containers(instance)
@@ -136,7 +138,7 @@ def check(self, instance):
         skipped_container_ids = self._report_containers_metrics(containers, instance)
 
         # Send events from Docker API
-        if instance.get('collect_events', True):
+        if _is_affirmative(instance.get('collect_events', True)):
             self._process_events(instance, ids_to_names, skipped_container_ids)
 
 
@@ -156,7 +158,7 @@ def _count_images(self, instance):
 
     def _get_and_count_containers(self, instance):
         tags = instance.get("tags", [])
-        with_size = instance.get('collect_container_size', False)
+        with_size = _is_affirmative(instance.get('collect_container_size', False))
 
         service_check_name = 'docker.service_up'
         try:
@@ -215,7 +217,7 @@ def _tags_match_patterns(self, tags, filters):
 
     def _report_containers_metrics(self, containers, instance):
         skipped_container_ids = []
-        collect_uncommon_metrics = instance.get("collect_all_metrics", False)
+        collect_uncommon_metrics = _is_affirmative(instance.get("collect_all_metrics", False))
         tags = instance.get("tags", [])
 
         # Pre-compile regex to include/exclude containers

conf.d/docker.yaml.example

@@ -65,3 +65,8 @@ instances:
     # Collect all the available cgroups metrics. All the relevant metrics are collected by default.
     #
     # collect_all_metrics: false
+
+    # Collect images stats
+    # Number of available active images and intermediate images as gauges
+    #
+    # collect_images_stats: true