Fix process start time

Previously we were recording process start time as seconds from boot.
This makes it so we record as epoch time.

changelogs/unreleased/63873-process-start-time.yml

+---
+title: Change ruby_process_start_time_seconds metric to unix timestamp instead of
+  seconds from boot.
+merge_request: 30195
+author: 
+type: fixed

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -104,7 +104,7 @@ Some basic Ruby runtime metrics are available:
 | ruby_process_cpu_seconds_total         | Gauge     | 12.0  | Total amount of CPU time per process |
 | ruby_process_max_fds                   | Gauge     | 12.0  | Maximum number of open file descriptors per process |
 | ruby_process_resident_memory_bytes     | Gauge     | 12.0  | Memory usage by process, measured in bytes |
-| ruby_process_start_time_seconds        | Gauge     | 12.0  | The elapsed time between system boot and the process started, measured in seconds |
+| ruby_process_start_time_seconds        | Gauge     | 12.0  | UNIX timestamp of process start time |
 
 [GC.stat]: https://ruby-doc.org/core-2.3.0/GC.html#method-c-stat
 

lib/gitlab/metrics/samplers/ruby_sampler.rb

@@ -6,6 +6,12 @@ module Gitlab
   module Metrics
     module Samplers
       class RubySampler < BaseSampler
+        def initialize(interval)
+          metrics[:process_start_time_seconds].set(labels.merge(worker_label), Time.now.to_i)
+
+          super
+        end
+
         def metrics
           @metrics ||= init_metrics
         end
@@ -47,7 +53,6 @@ module Gitlab
           metrics[:file_descriptors].set(labels.merge(worker_label), System.file_descriptor_count)
           metrics[:process_cpu_seconds_total].set(labels.merge(worker_label), ::Gitlab::Metrics::System.cpu_time)
           metrics[:process_max_fds].set(labels.merge(worker_label), ::Gitlab::Metrics::System.max_open_file_descriptors)
-          metrics[:process_start_time_seconds].set(labels.merge(worker_label), ::Gitlab::Metrics::System.process_start_time)
           set_memory_usage_metrics
           sample_gc
 

lib/gitlab/metrics/system.rb

@@ -31,14 +31,6 @@ module Gitlab
 
           match[1].to_i
         end
-
-        def self.process_start_time
-          fields = File.read('/proc/self/stat').split
-
-          # fields[21] is linux proc stat field "(22) starttime".
-          # The value is expressed in clock ticks, divide by clock ticks for seconds.
-          ( fields[21].to_i || 0 ) / clk_tck
-        end
       else
         def self.memory_usage
           0.0
@@ -51,10 +43,6 @@ module Gitlab
         def self.max_open_file_descriptors
           0
         end
-
-        def self.process_start_time
-          0
-        end
       end
 
       def self.cpu_time

spec/lib/gitlab/metrics/samplers/ruby_sampler_spec.rb

@@ -8,12 +8,19 @@ describe Gitlab::Metrics::Samplers::RubySampler do
     allow(Gitlab::Metrics::NullMetric).to receive(:instance).and_return(null_metric)
   end
 
+  describe '#initialize' do
+    it 'sets process_start_time_seconds' do
+      Timecop.freeze do
+        expect(sampler.metrics[:process_start_time_seconds].get).to eq(Time.now.to_i)
+      end
+    end
+  end
+
   describe '#sample' do
     it 'samples various statistics' do
       expect(Gitlab::Metrics::System).to receive(:cpu_time)
       expect(Gitlab::Metrics::System).to receive(:file_descriptor_count)
       expect(Gitlab::Metrics::System).to receive(:memory_usage)
-      expect(Gitlab::Metrics::System).to receive(:process_start_time)
       expect(Gitlab::Metrics::System).to receive(:max_open_file_descriptors)
       expect(sampler).to receive(:sample_gc)
 
@@ -44,13 +51,6 @@ describe Gitlab::Metrics::Samplers::RubySampler do
       sampler.sample
     end
 
-    it 'adds a metric containing the process start time' do
-      expect(Gitlab::Metrics::System).to receive(:process_start_time).and_return(12345)
-      expect(sampler.metrics[:process_start_time_seconds]).to receive(:set).with({}, 12345)
-
-      sampler.sample
-    end
-
     it 'adds a metric containing the process max file descriptors' do
       expect(Gitlab::Metrics::System).to receive(:max_open_file_descriptors).and_return(1024)
       expect(sampler.metrics[:process_max_fds]).to receive(:set).with({}, 1024)

spec/lib/gitlab/metrics/system_spec.rb

@@ -19,12 +19,6 @@ describe Gitlab::Metrics::System do
         expect(described_class.max_open_file_descriptors).to be > 0
       end
     end
-
-    describe '.process_start_time' do
-      it 'returns the process start time' do
-        expect(described_class.process_start_time).to be > 0
-      end
-    end
   else
     describe '.memory_usage' do
       it 'returns 0.0' do
@@ -43,12 +37,6 @@ describe Gitlab::Metrics::System do
         expect(described_class.max_open_file_descriptors).to eq(0)
       end
     end
-
-    describe 'process_start_time' do
-      it 'returns 0' do
-        expect(described_class.process_start_time).to eq(0)
-      end
-    end
   end
 
   describe '.cpu_time' do