Add validation of metric values

Make sure to validate that when adding metric samples that the value is
a number.

Fixes: https://gitlab.com/gitlab-org/gitlab-exporter/issues/60

lib/gitlab_exporter/database/tuple_stats.rb

@@ -35,7 +35,9 @@ module GitLab
 
           result.each do |table_name, tuple_stats|
             tuple_stats.each do |column_name, value|
-              @metrics.add("gitlab_database_stat_table_#{column_name}", value.to_s, table_name: table_name)
+              @metrics.add("gitlab_database_stat_table_#{column_name}",
+                           value,
+                           table_name: table_name) unless value.is_a?(Numeric)
             end
           end
 

lib/gitlab_exporter/prometheus.rb

@@ -16,6 +16,8 @@ module GitLab
       end
 
       def add(name, value, quantile = false, **labels)
+        fail "value must be a number" unless value.is_a?(Numeric)
+
         if quantile
           @quantiles[{ name: name, labels: labels }] << value
         else

spec/prometheus_metrics_spec.rb

@@ -14,4 +14,13 @@ describe GitLab::Exporter::PrometheusMetrics do
       /mymetric{mylabel="x",myotherlabel="y"} 1.3 \d*$/
     )
   end
+
+  it "fails to add a non-numeric metric value" do
+    expect {
+      subject.add("mymetric", "1.4", mylabel: "x", myotherlabel: "y").to_s
+    }.to raise_error(RuntimeError)
+    expect {
+      subject.add("mymetric", "invalid", mylabel: "x", myotherlabel: "y").to_s
+    }.to raise_error(RuntimeError)
+  end
 end