Fix RSpec expectation ordering error in metrics_spec callback test

[Copilot]

On certain CI matrix entries, collect_and_send invokes the callback during the first (probe) call, causing RSpec to reject a subsequent receive(:call) expectation on an already-invoked proc.

Change

• spec/elastic_apm/metrics_spec.rb: Replace the post-invocation RSpec message expectation with a plain call recorder (array spy). The callback appends each metricset to calls; after the second collect_and_send, the test asserts calls includes a Metricset. The next unless skip path for unsupported environments is preserved.

# Before — expectation set after callback may already have fired
subject.collect_and_send
next unless subject.sets.values.select { |s| s.metrics.any? }.any?
expect(callback).to receive(:call).with(Metricset).at_least(1)
subject.collect_and_send

# After — spy array avoids RSpec ordering constraint entirely
calls = []
callback = ->(metricset) { calls << metricset }
metrics = described_class.new(config, &callback)
begin
  metrics.start
  metrics.collect_and_send
  next unless metrics.sets.values.any? { |s| s.metrics.any? }
  calls.clear
  metrics.collect_and_send
  expect(calls).to include(Metricset)
ensure
  metrics.stop
end