Bump fluent-plugin-prometheus to 2.0.2

Metrics like emit_count, num_errors, and others are missing when 2.0.1
is used [1][2].

Also, bump .ruby-version to 2.7.4. It is the same as in the docker
image. The previous version is incompatible with some gems used in the
repo.

1. fluent/fluent-plugin-prometheus#195
2. fluent/fluent-plugin-prometheus@0e5d768

.ruby-version

@@ -1 +1 @@
-2.6.1
+2.7.4

docker/Gemfile

@@ -1,12 +1,12 @@
 source 'https://rubygems.org'
 
-# This is separate gemfile for building docker image that has all plugins 
+# This is separate gemfile for building docker image that has all plugins
 # for kubernetes log collection agent
 # List all required gems here and install via bundler to resolve dependencies
-gem "fluentd", ">=1.14.2" 
+gem "fluentd", ">=1.14.2"
 gem "fluent-plugin-systemd", "=1.0.2"
 gem "fluent-plugin-concat", "=2.4.0"
-gem "fluent-plugin-prometheus", "=2.0.1"
+gem "fluent-plugin-prometheus", "=2.0.2"
 gem "fluent-plugin-jq", "=0.5.1"
 gem "fluent-plugin-kubernetes_metadata_filter", ">=2.9.2"
 gem "oj", "=3.11.2"
@@ -20,4 +20,4 @@ gem "rack", ">= 2.1.4"
 gem "fluent-plugin-record-modifier", ">=2.1"
 
 
-gem 'fluent-plugin-splunk-hec', path: 'gem/'
+gem 'fluent-plugin-splunk-hec', path: 'gem/'

docker/Gemfile.lock

@@ -41,7 +41,7 @@ GEM
       fluentd (>= 0.14.0, < 1.15)
       kubeclient (>= 4.0.0, < 5.0.0)
       lru_redux
-    fluent-plugin-prometheus (2.0.1)
+    fluent-plugin-prometheus (2.0.2)
       fluentd (>= 1.9.1, < 2)
       prometheus-client (>= 2.1.0)
     fluent-plugin-record-modifier (2.1.0)
@@ -165,7 +165,7 @@ DEPENDENCIES
   fluent-plugin-concat (= 2.4.0)
   fluent-plugin-jq (= 0.5.1)
   fluent-plugin-kubernetes_metadata_filter (>= 2.9.2)
-  fluent-plugin-prometheus (= 2.0.1)
+  fluent-plugin-prometheus (= 2.0.2)
   fluent-plugin-record-modifier (>= 2.1)
   fluent-plugin-splunk-hec!
   fluent-plugin-systemd (= 1.0.2)
@@ -179,4 +179,4 @@ DEPENDENCIES
   rack (>= 2.1.4)
 
 BUNDLED WITH
-   2.2.32
+   2.3.11