Default enable_memory_uss_pss check to true + add docs

changelogs/unreleased/216597-drop-uss-pss-env-var.yml

+---
+title: Ruby metrics now include USS and PSS memory readings
+merge_request: 31707
+author:
+type: added

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -178,16 +178,18 @@ The following metrics are available:
 
 Some basic Ruby runtime metrics are available:
 
-| Metric                               | Type      | Since | Description |
-|:------------------------------------ |:--------- |:----- |:----------- |
-| `ruby_gc_duration_seconds`           | Counter   | 11.1  | Time spent by Ruby in GC |
-| `ruby_gc_stat_...`                   | Gauge     | 11.1  | Various metrics from [GC.stat](https://ruby-doc.org/core-2.6.5/GC.html#method-c-stat) |
-| `ruby_file_descriptors`              | Gauge     | 11.1  | File descriptors per process |
-| `ruby_sampler_duration_seconds`      | Counter   | 11.1  | Time spent collecting stats |
-| `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 |
-| `ruby_process_start_time_seconds`    | Gauge     | 12.0  | UNIX timestamp of process start time |
+| Metric                                   | Type      | Since | Description |
+|:---------------------------------------- |:--------- |:----- |:----------- |
+| `ruby_gc_duration_seconds`               | Counter   | 11.1  | Time spent by Ruby in GC |
+| `ruby_gc_stat_...`                       | Gauge     | 11.1  | Various metrics from [GC.stat](https://ruby-doc.org/core-2.6.5/GC.html#method-c-stat) |
+| `ruby_file_descriptors`                  | Gauge     | 11.1  | File descriptors per process |
+| `ruby_sampler_duration_seconds`          | Counter   | 11.1  | Time spent collecting stats |
+| `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 (RSS/Resident Set Size) |
+| `ruby_process_unique_memory_bytes`       | Gauge     | 13.0  | Memory usage by process (USS/Unique Set Size) |
+| `ruby_process_proportional_memory_bytes` | Gauge     | 13.0  | Memory usage by process (PSS/Proportional Set Size) |
+| `ruby_process_start_time_seconds`        | Gauge     | 12.0  | UNIX timestamp of process start time |
 
 ## Unicorn Metrics
 

lib/gitlab/metrics/samplers/ruby_sampler.rb

@@ -88,7 +88,7 @@ module Gitlab
         def set_memory_usage_metrics
           metrics[:process_resident_memory_bytes].set(labels, System.memory_usage_rss)
 
-          if Gitlab::Utils.to_boolean(ENV['enable_memory_uss_pss'])
+          if Gitlab::Utils.to_boolean(ENV['enable_memory_uss_pss'] || '1')
             memory_uss_pss = System.memory_usage_uss_pss
             metrics[:process_unique_memory_bytes].set(labels, memory_uss_pss[:uss])
             metrics[:process_proportional_memory_bytes].set(labels, memory_uss_pss[:pss])

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

@@ -8,7 +8,6 @@ describe Gitlab::Metrics::Samplers::RubySampler do
 
   before do
     allow(Gitlab::Metrics::NullMetric).to receive(:instance).and_return(null_metric)
-    stub_env('enable_memory_uss_pss', "1")
   end
 
   describe '#initialize' do
@@ -37,6 +36,21 @@ describe Gitlab::Metrics::Samplers::RubySampler do
       sampler.sample
     end
 
+    context 'when USS+PSS sampling is disabled via environment' do
+      before do
+        stub_env('enable_memory_uss_pss', "0")
+      end
+
+      it 'does not sample USS or PSS' do
+        expect(Gitlab::Metrics::System).not_to receive(:memory_usage_uss_pss)
+
+        expect(sampler.metrics[:process_unique_memory_bytes]).not_to receive(:set)
+        expect(sampler.metrics[:process_proportional_memory_bytes]).not_to receive(:set)
+
+        sampler.sample
+      end
+    end
+
     it 'adds a metric containing the amount of open file descriptors' do
       expect(Gitlab::Metrics::System).to receive(:file_descriptor_count)
                                            .and_return(4)