Try to protect against using undef as a label value

This is surprisingly easy to do when generating metrics.

Changes

@@ -1,6 +1,7 @@
 Revision history for {{$dist->name}}
 
 {{$NEXT}}
+- Warn and handle attempts to set label values to undef
 
 0.010     2022-02-01 08:25:56+11:00 Australia/Melbourne
 - Warn and handle attempts to set or add non-numbers

lib/Prometheus/Tiny.pm

@@ -36,9 +36,14 @@ sub _format_labels {
 
   join ',', map {
     my $lv = $to_format->{$_};
-    $lv =~ s/(["\\])/\\$1/sg;
-    $lv =~ s/\n/\\n/sg;
-    qq{$_="$lv"}
+    if (defined $lv) {
+      $lv =~ s/(["\\])/\\$1/sg;
+      $lv =~ s/\n/\\n/sg;
+      qq{$_="$lv"}
+    } else {
+      carp "label '$_' has an undefined value, dropping it";
+      ()
+    }
   } sort keys %$to_format;
 }
 

t/50-warn-undef.t

@@ -32,4 +32,16 @@ some_metric 10
 EOF
 }
 
+{
+  my $p = Prometheus::Tiny->new;
+  $p->set('some_metric', 10);
+  warning_like
+    { $p->add('some_metric', 10, { foo => undef }) }
+    qr/label '.+' has an undefined value, dropping it/,
+    'undef label value emits a warning';
+  is $p->format, <<EOF, 'add metric undef label formatted correctly';
+some_metric 20
+EOF
+}
+
 done_testing;