Support :all as an aggregation mode in DirectFileStore

README.md

@@ -256,6 +256,15 @@ class MyComponent
 end
 ```
 
+### Reserved labels
+
+The following labels are reserved by the client library, and attempting to use them in a
+metric definition will result in a
+`Prometheus::Client::LabelSetValidator::ReservedLabelError` being raised:
+
+  - `:job`
+  - `:instance`
+  - `:pid`
 
 ## Data Stores
 
@@ -362,7 +371,8 @@ summing the values of each process.
 
 For Gauges, however, this may not be the right thing to do, depending on what they're 
 measuring. You might want to take the maximum or minimum value observed in any process,
-rather than the sum of all of them.
+rather than the sum of all of them. You may also want to export each process's individual
+value.
 
 In those cases, you should use the `store_settings` parameter when registering the 
 metric, to specify an `:aggregation` setting. 

lib/prometheus/client/data_stores/direct_file_store.rb

@@ -26,7 +26,7 @@ module DataStores
 
       class DirectFileStore
         class InvalidStoreSettingsError < StandardError; end
-        AGGREGATION_MODES = [MAX = :max, MIN = :min, SUM = :sum]
+        AGGREGATION_MODES = [MAX = :max, MIN = :min, SUM = :sum, ALL = :all]
         DEFAULT_METRIC_SETTINGS = { aggregation: SUM }
 
         def initialize(dir:)
@@ -140,6 +140,10 @@ def in_process_sync
           end
 
           def store_key(labels)
+            if @values_aggregation_mode == ALL
+              labels[:pid] = process_id
+            end
+
             labels.map{|k,v| "#{CGI::escape(k.to_s)}=#{CGI::escape(v.to_s)}"}.join('&')
           end
 
@@ -168,6 +172,8 @@ def aggregate_values(values)
               values.max
             elsif @values_aggregation_mode == MIN
               values.min
+            elsif @values_aggregation_mode == ALL
+              values.first
             else
               raise InvalidStoreSettingsError,
                     "Invalid Aggregation Mode: #{ @values_aggregation_mode }"

lib/prometheus/client/label_set_validator.rb

@@ -6,7 +6,7 @@ module Client
     # Prometheus specification.
     class LabelSetValidator
       # TODO: we might allow setting :instance in the future
-      BASE_RESERVED_LABELS = [:job, :instance].freeze
+      BASE_RESERVED_LABELS = [:job, :instance, :pid].freeze
 
       class LabelSetError < StandardError; end
       class InvalidLabelSetError < LabelSetError; end

spec/prometheus/client/data_stores/direct_file_store_spec.rb

@@ -150,6 +150,42 @@
     end
   end
 
+  context "with a metric that takes ALL instead of SUM" do
+    it "reports all the values from different processes" do
+      allow(Process).to receive(:pid).and_return(12345)
+      metric_store1 = subject.for_metric(
+        :metric_name,
+        metric_type: :gauge,
+        metric_settings: { aggregation: :all }
+      )
+      metric_store1.set(labels: { foo: "bar" }, val: 1)
+      metric_store1.set(labels: { foo: "baz" }, val: 7)
+      metric_store1.set(labels: { foo: "yyy" }, val: 3)
+
+      allow(Process).to receive(:pid).and_return(23456)
+      metric_store2 = subject.for_metric(
+        :metric_name,
+        metric_type: :gauge,
+        metric_settings: { aggregation: :all }
+      )
+      metric_store2.set(labels: { foo: "bar" }, val: 3)
+      metric_store2.set(labels: { foo: "baz" }, val: 2)
+      metric_store2.set(labels: { foo: "zzz" }, val: 1)
+
+      expect(metric_store1.all_values).to eq(
+        { foo: "bar", pid: "12345" } => 1.0,
+        { foo: "bar", pid: "23456" } => 3.0,
+        { foo: "baz", pid: "12345" } => 7.0,
+        { foo: "baz", pid: "23456" } => 2.0,
+        { foo: "yyy", pid: "12345" } => 3.0,
+        { foo: "zzz", pid: "23456" } => 1.0,
+      )
+
+      # Both processes should return the same value
+      expect(metric_store1.all_values).to eq(metric_store2.all_values)
+    end
+  end
+
   it "resizes the File if metrics get too big" do
      truncate_calls_count = 0
      allow_any_instance_of(Prometheus::Client::DataStores::DirectFileStore::FileMappedDict).

spec/prometheus/client/label_set_validator_spec.rb

@@ -37,7 +37,7 @@
     end
 
     it 'raises ReservedLabelError if a label key is reserved' do
-      [:job, :instance].each do |label|
+      [:job, :instance, :pid].each do |label|
         expect do
           validator.validate_symbols!(label => 'value')
         end.to raise_exception(described_class::ReservedLabelError)