Update Prometheus client to 0.15.0

This includes a bugfix for label ordering: if metrics were reported
with a different label ordering than they were initialized with, they
would not get stored in the file and thus not reported when scraped.

Diff in
https://my.diffend.io/gems/prometheus-client-mmap/0.14.0/0.15.0

Merge request with the changes
https://gitlab.com/gitlab-org/prometheus-client-mmap/-/merge_requests/60

Gemfile

@@ -333,7 +333,7 @@ gem 'snowplow-tracker', '~> 0.6.1'
 # Metrics
 gem 'method_source', '~> 1.0', require: false
 gem 'webrick', '~> 1.6.1', require: false
-gem 'prometheus-client-mmap', '~> 0.14.0', require: 'prometheus/client'
+gem 'prometheus-client-mmap', '~> 0.15.0', require: 'prometheus/client'
 
 gem 'warning', '~> 1.2.0'
 

Gemfile.lock

@@ -925,7 +925,7 @@ GEM
       coderay
       parser
       unparser
-    prometheus-client-mmap (0.14.0)
+    prometheus-client-mmap (0.15.0)
     pry (0.13.1)
       coderay (~> 1.1)
       method_source (~> 1.0)
@@ -1574,7 +1574,7 @@ DEPENDENCIES
   pg_query (~> 2.1)
   png_quantizator (~> 0.2.1)
   premailer-rails (~> 1.10.3)
-  prometheus-client-mmap (~> 0.14.0)
+  prometheus-client-mmap (~> 0.15.0)
   pry-byebug
   pry-rails (~> 0.3.9)
   pry-shell (~> 0.4.0)