Remove :puma_phase metrics

We don't use phase restarts, as we use `preload_app`:
https://github.com/puma/puma/blob/master/README.md#clustered-mode
`:puma_phase` values will always be zero.

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -120,7 +120,6 @@ When Puma is used instead of Unicorn, following metrics are available:
 | puma_workers                                 | Gauge   | 12.0  | Total number of workers |
 | puma_running_workers                         | Gauge   | 12.0  | Number of booted workers |
 | puma_stale_workers                           | Gauge   | 12.0  | Number of old workers |
-| puma_phase                                   | Gauge   | 12.0  | Phase number (increased during phased restarts) |
 | puma_running                                 | Gauge   | 12.0  | Number of running threads |
 | puma_queued_connections                      | Gauge   | 12.0  | Number of connections in that worker's "todo" set waiting for a worker thread |
 | puma_active_connections                      | Gauge   | 12.0  | Number of threads processing a request |

lib/gitlab/metrics/samplers/puma_sampler.rb

@@ -15,7 +15,6 @@ module Gitlab
             puma_workers:            ::Gitlab::Metrics.gauge(:puma_workers, 'Total number of workers'),
             puma_running_workers:    ::Gitlab::Metrics.gauge(:puma_running_workers, 'Number of active workers'),
             puma_stale_workers:      ::Gitlab::Metrics.gauge(:puma_stale_workers, 'Number of stale workers'),
-            puma_phase:              ::Gitlab::Metrics.gauge(:puma_phase, 'Phase number (increased during phased restarts)'),
             puma_running:            ::Gitlab::Metrics.gauge(:puma_running, 'Number of running threads'),
             puma_queued_connections: ::Gitlab::Metrics.gauge(:puma_queued_connections, 'Number of connections in that worker\'s "todo" set waiting for a worker thread'),
             puma_active_connections: ::Gitlab::Metrics.gauge(:puma_active_connections, 'Number of threads processing a request'),
@@ -54,7 +53,6 @@ module Gitlab
             last_status = worker['last_status']
             labels = { worker: "worker_#{worker['index']}" }
 
-            metrics[:puma_phase].set(labels, worker['phase'])
             set_worker_metrics(last_status, labels) if last_status.present?
           end
         end
@@ -76,7 +74,6 @@ module Gitlab
           metrics[:puma_workers].set(labels, stats['workers'])
           metrics[:puma_running_workers].set(labels, stats['booted_workers'])
           metrics[:puma_stale_workers].set(labels, stats['old_workers'])
-          metrics[:puma_phase].set(labels, stats['phase'])
         end
 
         def set_worker_metrics(stats, labels = {})

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

@@ -46,8 +46,6 @@ describe Gitlab::Metrics::Samplers::PumaSampler do
         expect(subject.metrics[:puma_workers]).to receive(:set).with(labels, 2)
         expect(subject.metrics[:puma_running_workers]).to receive(:set).with(labels, 2)
         expect(subject.metrics[:puma_stale_workers]).to receive(:set).with(labels, 0)
-        expect(subject.metrics[:puma_phase]).to receive(:set).once.with(labels, 2)
-        expect(subject.metrics[:puma_phase]).to receive(:set).once.with({ worker: 'worker_0' }, 1)
 
         subject.sample
       end