Make auto detection of CPUs in default puma config optional, not default

Fixes rails#52522

Using `Concurrent.available_processor_count` helper might result in
incorrect configurations on some cloud hosts with shared CPUs or
platforms that inaccurately report CPU counts.

Changes:
* Make puma.rb spawn only 1 worker by default if WEB_CONCURRENCY is not set
* Modify default puma.rb to offer "auto" as an option for WEB_CONCURRENCY
* Enhance comment in default deploy.yml to offer "auto" as an option for WEB_CONCURRENCY
* Update note to tuning_performance_for_deployment.md to explain the WEB_CONCURRENCY

Author: denys-chaikovskyi

guides/source/tuning_performance_for_deployment.md

@@ -121,8 +121,17 @@ If you use more than one thread per process, then it should be set to how many
 or if the server is running multiple applications, to how many cores you want the application to use.
 If you only use one thread per worker, then you can increase it to above one per process to account for when workers are
 idle waiting for I/O operations.
-In the default generated configuration, it is set to use all the available processor cores on the server via the
-`Concurrent.available_processor_count` helper. You can also modify it by setting the `WEB_CONCURRENCY` environment variable.
+
+In the default generated configuration, it is set to use 1 worker.
+You can also modify it by setting the `WEB_CONCURRENCY` environment variable.
+
+* Setting `WEB_CONCURRENCY` to a specific number will configure Puma to use that many workers.
+* Setting `WEB_CONCURRENCY` to "auto" will make Puma use all available processor cores on the server, as determined by
+  the `Concurrent.available_processor_count` helper.
+
+Please note that using "auto" might result in incorrect configurations on some cloud hosts with shared CPUs or platforms
+that inaccurately report CPU counts.
+
 
 ### YJIT
 

railties/lib/rails/generators/rails/app/templates/config/deploy.yml.tt

@@ -30,7 +30,7 @@ env:
     - RAILS_MASTER_KEY
   clear:
     # Set this to the number of cores you wish the application to use on each server.
-    # If not set, app will attempt to use all available cores (but may guess wrong on some cloud hosts!)
+    # You can use "auto" and the app will attempt to use all available cores (but may guess wrong on some cloud hosts!)
     WEB_CONCURRENCY: 1
 
     # Match this to the database server to configure Active Record correctly

railties/lib/rails/generators/rails/app/templates/config/puma.rb.tt

@@ -32,9 +32,14 @@ when "production"
   # If you are running more than 1 thread per process, the workers count
   # should be equal to the number of processors (CPU cores) in production.
   #
-  # Automatically detect the number of available processors in production.
+  # Automatically detect the number of available processors in production
+  # when WEB_CONCURRENCY is set to "auto".
   require "concurrent-ruby"
-  workers_count = Integer(ENV.fetch("WEB_CONCURRENCY") { Concurrent.available_processor_count })
+  workers_count = if ENV["WEB_CONCURRENCY"] == "auto"
+                    Concurrent.available_processor_count
+                  else
+                    Integer(ENV.fetch("WEB_CONCURRENCY", 1))
+                  end
   workers workers_count if workers_count > 1
 
   preload_app!