cmd/soroban-rpc/internal/events: Fix eventsDurationMetric #184

Job	Run time
Build (ubuntu-latest, x86_64-unknown-linux-gnu, amd64)	2m 26s
Build (ubuntu-latest, aarch64-unknown-linux-gnu, arm64)	2m 59s
Integration tests (ubuntu-20.04, 1.22, .CLI.)	3m 58s
Build (macos-latest, x86_64-apple-darwin, amd64)	3m 55s
Integration tests (ubuntu-20.04, 1.22, ^Test(([^C])\|(C[^L])\|(CL[^I])).*$)	13m 25s
Unit tests (ubuntu-20.04, 1.22)	2m 8s
Build (macos-latest, aarch64-apple-darwin, arm64)	5m 38s
Unit tests (ubuntu-22.04, 1.22)	2m 9s
Build (windows-latest, x86_64-pc-windows-gnu, amd64)	7m 17s
Integration tests (ubuntu-22.04, 1.22, .CLI.)	4m 29s
Integration tests (ubuntu-22.04, 1.22, ^Test(([^C])\|(C[^L])\|(CL[^I])).*$)	13m 32s
	1h 1m 56s

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