Prevent prometheus_quantile_summary crash without datapoints

Quantile evaluation for metrics prior to observe/_ is not a valid operation.
Any metric without observation will now be skipped until data are
provided.

src/metrics/prometheus_quantile_summary.erl

@@ -304,9 +304,12 @@ values(Registry, Name) ->
 
       MFValues = load_all_values(Registry, Name),
       ReducedMap = lists:foldl(
-        fun([L, C, S, QE], ResAcc) ->
-          {PrevCount, PrevSum, PrevQE} = maps:get(L, ResAcc, {0, 0, quantile(Configuration)}),
-          ResAcc#{L => {PrevCount + C, PrevSum + S, quantile_merge(PrevQE, QE)}}
+        fun
+          ([_, 0, _, _], ResAcc) ->
+            ResAcc; %% Ignore quantile evaluation if no data are provided
+          ([L, C, S, QE], ResAcc) ->
+            {PrevCount, PrevSum, PrevQE} = maps:get(L, ResAcc, {0, 0, quantile(Configuration)}),
+            ResAcc#{L => {PrevCount + C, PrevSum + S, quantile_merge(PrevQE, QE)}}
         end,
       #{},
       MFValues),