Update prometheus-client-mmap to v0.12.0

This removes the deprecated rb_safe_level() and rb_secure() calls. Ruby
2.7 removed the the taint checking mechanism because "the taint
mechanism is too expensive to maintain for the merit of it.":
https://bugs.ruby-lang.org/issues/16131

This pulls in
https://gitlab.com/gitlab-org/prometheus-client-mmap/-/merge_requests/53

Gemfile

@@ -330,7 +330,7 @@ group :metrics do
   gem 'method_source', '~> 1.0', require: false
 
   # Prometheus
-  gem 'prometheus-client-mmap', '~> 0.11.0'
+  gem 'prometheus-client-mmap', '~> 0.12.0'
   gem 'raindrops', '~> 0.18'
 end
 

Gemfile.lock

@@ -838,7 +838,7 @@ GEM
       parser
       unparser
     procto (0.0.3)
-    prometheus-client-mmap (0.11.0)
+    prometheus-client-mmap (0.12.0)
     pry (0.13.1)
       coderay (~> 1.1)
       method_source (~> 1.0)
@@ -1419,7 +1419,7 @@ DEPENDENCIES
   pg (~> 1.1)
   png_quantizator (~> 0.2.1)
   premailer-rails (~> 1.10.3)
-  prometheus-client-mmap (~> 0.11.0)
+  prometheus-client-mmap (~> 0.12.0)
   pry-byebug (~> 3.9.0)
   pry-rails (~> 0.3.9)
   rack (~> 2.0.9)