test(integration): wait for fluent-bit socket release in TestMetricsPortOverrideEnv

[rafaelwestphal]

Description

In TestMetricsPortOverrideEnv on Linux, the test stops the Ops Agent service, writes the systemd drop-in override.conf files to remap ports to 40001/40002, and immediately starts the service.
Because google-cloud-ops-agent.service is a Type=oneshot parent unit, stopping it causes systemd to initiate asynchronous graceful teardown of google-cloud-ops-agent-fluent-bit.service. Fluent Bit has a 5-second shutdown grace period to flush buffers and close connections before releasing its socket.

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