New check for indices that have too many nested fields

[majormoses]

In ES5 there are new limits, see: elastic/elasticsearch#14983

[jgough]

I believe this patch fixes the issue

diff --git a/bin/check-es-indices-field-count.rb b/bin/check-es-indices-field-count.rb
index 55deea8..2e17a3a 100755
--- a/bin/check-es-indices-field-count.rb
+++ b/bin/check-es-indices-field-count.rb
@@ -127,7 +127,7 @@ class ESIndicesFieldCount < Sensu::Plugin::Check::CLI
           fieldcount = if type_mapping['properties'].nil?
                          0
                        else
-                         type_mapping['properties'].length
+                         countnestedfields(type_mapping['properties'])
                        end
           type_field_count[type] = fieldcount
         end
@@ -139,6 +139,19 @@ class ESIndicesFieldCount < Sensu::Plugin::Check::CLI
     index_field_count
   end

+  def countnestedfields(fields)
+    fieldcount = 0
+    fields.each do |field, value|
+      fieldcount += if value.key?('properties')
+                      countnestedfields(value['properties'])
+                    else
+                      1
+                    end
+      end
+
+      fieldcount
+  end
+
   def fieldlimitsetting
     field_limit_setting = {}
     settings = client.indices.get_settings index: config[:index]