Add /metrics endpoint for Prometheus scraping #18

Job	Run time
Create Release	7s
Build and Push Docker Image	7m 16s
Build Release (macos-latest, aarch64-apple-darwin, diffscope, diffscope-aarch64-apple-darwin)	3m 13s
Build Release (ubuntu-latest, x86_64-unknown-linux-musl, diffscope, diffscope-x86_64-unknown-linu...	2m 41s
Build Release (ubuntu-latest, aarch64-unknown-linux-gnu, diffscope, diffscope-aarch64-unknown-lin...	2m 58s
Build Release (ubuntu-latest, x86_64-unknown-linux-gnu, diffscope, diffscope-x86_64-unknown-linux...	2m 26s
Build Release (windows-latest, x86_64-pc-windows-msvc, diffscope.exe, diffscope-x86_64-pc-windows...	5m 35s
Build Release (macos-latest, x86_64-apple-darwin, diffscope, diffscope-x86_64-apple-darwin)	2m 52s
	27m 8s

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