feat(layers): Use aggregation to count records

This project previously depended on Elasticsearch types in order to
provide a count of the number of records in each layer.

Elasticsearch types will be going away in Elasticsearch 6, and are
already deprecated in ES5.

Instead of relying on types, the count of records per layer is now
created using a [terms aggregation](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-terms-aggregation.html)
similar to the source and layers [autodetection
code](pelias/api#1316) recently added to
pelias/api.

This is one of the last few roadblocks for dropping Pelias's use of
types completely and merging PRs like
pelias/schema#293 that will drastically simplify
our schema.

Connects pelias/pelias#461
Connects pelias/pelias#719

jobs/counts.rb

@@ -2,7 +2,11 @@
 require_relative 'include.rb'
 
 SCHEDULER.every '30s' do
-  types = %w(
+
+  # make a list of common layers
+  # total is also first to ensure it shows up first
+  default_layers = %w(
+    total
     address
     venue
     street
@@ -18,20 +22,43 @@
     region
     postalcode
   )
-  types_counts = Hash.new(value: 0)
-
-  # get total
-  total_url = URI.parse "#{@es_endpoint}#{@es_index}/_stats/docs"
-  total_response = JSON.parse Net::HTTP.get_response(total_url).body
-  total_count = as_val(total_response['indices'][@es_index]['primaries']['docs']['count'])
-  types_counts['total'] = { label: 'total', value: total_count }
-
-  # get types
-  types.each do |t|
-    url = URI.parse "#{@es_endpoint}#{@es_index}/#{t}/_count"
-    response = JSON.parse Net::HTTP.get_response(url).body
-    count = as_val(response['count'])
-    types_counts[t] = { label: t, value: count }
+  layer_counts = Hash.new(value: 0)
+
+  # initialize type counts to 0 for common layers
+  default_layers.each do |type|
+    layer_counts[type] = { label: type, value: 0 }
   end
-  send_event('types-counts', items: types_counts.values)
+
+  # aggregation query to get all layer counts
+  query = {
+    aggs: {
+      layers: {
+        terms: {
+          field: 'layer',
+          size: 1000
+        }
+      }
+    },
+    size: 0
+  }
+
+  # get layer counts by aggregation
+  url = URI.parse "#{@es_endpoint}#{@es_index}/_search"
+
+  response = Net::HTTP.post(url, query.to_json)
+
+  response_body = JSON.parse response.body
+
+  # set total count
+  layer_counts['total'] = { label: 'total', value: as_val(response_body['hits']['total']) }
+
+  layer_count_results = response_body['aggregations']['layers']['buckets']
+
+  layer_count_results.each do |result|
+    layer = result['key']
+    count = as_val(result['doc_count'])
+    layer_counts[layer] = { label: layer, value: count }
+  end
+
+  send_event('types-counts', items: layer_counts.values)
 end