Use unfrozen version of Symbol#to_s (#132)

Use unfrozen version of Symbol#to_s (#132)

Makes it compatible with Shopify/symbol-fstring

Co-authored-by: f f@sutty.nl

diff --git a/lib/prometheus_exporter/instrumentation/active_record.rb b/lib/prometheus_exporter/instrumentation/active_record.rb
index 174f9a3..03b21df 100644
--- a/lib/prometheus_exporter/instrumentation/active_record.rb
+++ b/lib/prometheus_exporter/instrumentation/active_record.rb
@@ -69,7 +69,7 @@ module PrometheusExporter::Instrumentation
 
         labels_from_config = pool.spec.config
           .select { |k, v| @config_labels.include? k }
-          .map { |k, v| [k.to_s.prepend("dbconfig_"), v] }
+          .map { |k, v| [k.to_s.dup.prepend("dbconfig_"), v] }
 
         labels = @metric_labels.merge(pool_name: pool.spec.name).merge(Hash[labels_from_config])
 

GitHub sha: f63435cb

This commit appears in #132 which was merged by SamSaffron.