Do now allow DirectFileStore#increment when using :most_recent aggregation

If we do this, we'd be incrementing the value for *this* process, not the
global one, which is almost certainly not what the user wants to do.

This is not very pretty because we may end up raising an exception in
production (as test/dev tend to not use DirectFileStore), but we consider
it better than letting the user mangle their numbers and end up with
incorrect metrics.

Signed-off-by: Daniel Magliola <danielmagliola@gocardless.com>

Author: Daniel Magliola

README.md

@@ -351,6 +351,9 @@ Counters, Histograms and Summaries are `SUM`med, and Gauges report all their val
 for each process), tagged with a `pid` label. You can also select `SUM`, `MAX`, `MIN`, or
 `MOST_RECENT` for your gauges, depending on your use case.
 
+Please note that that the `MOST_RECENT` aggregation only works for gauges, and it does not
+allow the use of `increment` / `decrement`, you can only use `set`. 
+
 **Memory Usage**: When scraped by Prometheus, this store will read all these files, get all
 the values and aggregate them. We have notice this can have a noticeable effect on memory
 usage for your app. We recommend you test this in a realistic usage scenario to make sure

lib/prometheus/client/data_stores/direct_file_store.rb

@@ -105,6 +105,12 @@ def set(labels:, val:)
           end
 
           def increment(labels:, by: 1)
+            if @values_aggregation_mode == DirectFileStore::MOST_RECENT
+              raise InvalidStoreSettingsError,
+                    "The :most_recent aggregation does not support the use of increment"\
+                      "/decrement"
+            end
+
             key = store_key(labels)
             in_process_sync do
               value = internal_store.read_value(key)

spec/prometheus/client/data_stores/direct_file_store_spec.rb

@@ -329,6 +329,18 @@
       # Both processes should return the same value
       expect(metric_store1.all_values).to eq(metric_store2.all_values)
     end
+
+    it "does now allow `increment`, only `set`" do
+      metric_store1 = subject.for_metric(
+        :metric_name,
+        metric_type: :gauge,
+        metric_settings: { aggregation: :most_recent }
+      )
+
+      expect do
+        metric_store1.increment(labels: {})
+      end.to raise_error(Prometheus::Client::DataStores::DirectFileStore::InvalidStoreSettingsError)
+    end
   end
 
   it "resizes the File if metrics get too big" do