Add emulator handler for EmitMetric with MetricStore collection

[Mielek]

Summary

Implement EmitMetricHandler that collects emitted metrics into a MetricStore for test assertions.

Changes

• New: src/Testing/Emulator/Data/EmittedMetric.cs - record for collected metrics
• New: src/Testing/Emulator/Data/MetricStore.cs - store with ImmutableArray access
• New: test/Test.Testing/Emulator/Policies/EmitMetricTests.cs - 6 test cases
• Modified: handler, GatewayContext, TestDocumentExtensions

Usage

var test = new MyDocument().AsTestDocument();
test.RunInbound();
var metrics = test.SetupMetricStore().Metrics;
metrics.Should().HaveCount(1);
metrics[0].Name.Should().Be("my-metric");