test: make the runs the after_hook after forking more predictable

[xuan-cao-swi]

Description

metrics-sdk test case fails due to a race condition in the meter_provider initialization, which causes the runs the after_hook after forking test to fail.

  1) Error:
OpenTelemetry::SDK::Metrics::ForkHooks#test_0001_runs the after_hook after forking:
NoMethodError: undefined method 'metric_readers' for an instance of OpenTelemetry::Internal::ProxyMeterProvider
    lib/opentelemetry/sdk/metrics/fork_hooks.rb:21:in 'OpenTelemetry::SDK::Metrics::ForkHooks.after_fork'
    lib/opentelemetry/sdk/metrics/fork_hooks.rb:29:in 'block in OpenTelemetry::SDK::Metrics::ForkHooks#_fork'
    <internal:kernel>:91:in 'Kernel#tap'
    lib/opentelemetry/sdk/metrics/fork_hooks.rb:28:in 'OpenTelemetry::SDK::Metrics::ForkHooks#_fork'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:15:in 'Kernel#fork'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:15:in 'block in fork_with_fork_hooks'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:45:in 'with_pipe'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:14:in 'fork_with_fork_hooks'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:58:in 'block (3 levels) in <top (required)>'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:45:in 'with_pipe'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:52:in 'block (2 levels) in <top (required)>'

207 runs, 16653 assertions, 0 failures, 1 errors, 0 skips
........

Finished in 41.416792s, 4.9980 runs/s, 402.0833 assertions/s.

  1) Error:
OpenTelemetry::SDK::Metrics::ForkHooks#test_0001_runs the after_hook after forking:
Timeout::Error: execution expired
    /opt/hostedtoolcache/Ruby/3.4.1/x64/lib/ruby/3.4.0/timeout.rb:40:in 'Timeout::Error.handle_timeout'
    /opt/hostedtoolcache/Ruby/3.4.1/x64/lib/ruby/3.4.0/timeout.rb:194:in 'Timeout.timeout'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:26:in 'block in fork_with_fork_hooks'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:45:in 'with_pipe'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:14:in 'fork_with_fork_hooks'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:58:in 'block (3 levels) in <top (required)>'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:45:in 'with_pipe'
    test/opentelemetry/sdk/metrics/fork_hooks_test.rb:52:in 'block (2 levels) in <top (required)>'

207 runs, 16653 assertions, 0 failures, 1 errors, 0 skips

This fix aims to stub the meter_provider with an initialized one. However, we may need to consider improving the method self.after_fork to make it more robust.

def self.after_fork
  meter_provider = ::OpenTelemetry.meter_provider
  return unless meter_provider.respond_to?(:metric_readers)
  
  meter_provider.metric_readers.each do |reader|
     reader.after_fork if reader.respond_to?(:after_fork)
  end
end