Bump prometheus-client-mmap to 0.9.8

This fixes a critical error where a nil exception would be hit
if an error occurred while logging a Prometheus metric:

https://gitlab.com/gitlab-org/prometheus-client-mmap/merge_requests/36

Gemfile

@@ -309,7 +309,7 @@ group :metrics do
   gem 'influxdb', '~> 0.2', require: false
 
   # Prometheus
-  gem 'prometheus-client-mmap', '~> 0.9.7'
+  gem 'prometheus-client-mmap', '~> 0.9.8'
   gem 'raindrops', '~> 0.18'
 end
 

Gemfile.lock

@@ -652,7 +652,7 @@ GEM
       parser
       unparser
     procto (0.0.3)
-    prometheus-client-mmap (0.9.7)
+    prometheus-client-mmap (0.9.8)
     pry (0.11.3)
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
@@ -1173,7 +1173,7 @@ DEPENDENCIES
   peek-redis (~> 1.2.0)
   pg (~> 1.1)
   premailer-rails (~> 1.9.7)
-  prometheus-client-mmap (~> 0.9.7)
+  prometheus-client-mmap (~> 0.9.8)
   pry-byebug (~> 3.5.1)
   pry-rails (~> 0.3.4)
   puma (~> 3.12)