feat(metrics): add observable gauge for batch queue worker length #4002

Job	Run time
typecheck / typecheck	5m 53s
e2e / 🧪 CLI v3 tests (windows-latest - npm)	4m 7s
e2e / 🧪 CLI v3 tests (windows-latest - pnpm)	3m 33s
e2e / 🧪 CLI v3 tests (ubuntu-latest - npm)	1m 44s
e2e / 🧪 CLI v3 tests (ubuntu-latest - pnpm)	1m 47s
units / packages / 🧪 Unit Tests: Packages (1, 1)	7m 58s
units / internal / 🧪 Unit Tests: Internal (4, 8)	4m 34s
units / webapp / 🧪 Unit Tests: Webapp (1, 8)	5m 38s
units / internal / 🧪 Unit Tests: Internal (6, 8)	4m 28s
units / webapp / 🧪 Unit Tests: Webapp (2, 8)	3m 55s
units / webapp / 🧪 Unit Tests: Webapp (6, 8)	2m 15s
units / internal / 🧪 Unit Tests: Internal (5, 8)	3m 37s
units / webapp / 🧪 Unit Tests: Webapp (7, 8)	5m 43s
units / webapp / 🧪 Unit Tests: Webapp (4, 8)	3m 28s
units / internal / 🧪 Unit Tests: Internal (8, 8)	2m 37s
units / internal / 🧪 Unit Tests: Internal (2, 8)	4m 55s
units / webapp / 🧪 Unit Tests: Webapp (5, 8)	7m 22s
units / internal / 🧪 Unit Tests: Internal (3, 8)	5m 24s
units / webapp / 🧪 Unit Tests: Webapp (8, 8)	2m 23s
units / internal / 🧪 Unit Tests: Internal (1, 8)	5m 40s
units / webapp / 🧪 Unit Tests: Webapp (3, 8)	5m 51s
units / internal / 🧪 Unit Tests: Internal (7, 8)	4m 40s
units / internal / 📊 Merge Reports	22s
units / webapp / 📊 Merge Reports	43s
units / packages / 📊 Merge Reports	21s
	1h 38m 58s

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