NATS source: Allow vector to start with unavailable source

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Use Cases

Configuring NATS sources, I would still like my vector to run if one of them is unavailable.

Currently, it is able to support a service going down temporary after startup, but it fails to start if one is unavailable.

Attempted Solutions

Searched for any configuration that would allow, but did not find any.

Proposal

I would love to be able to already configure my vector sources, and still have it running and handling logs/metrics/... even if one or more sources are not available at start.

Just retry them at times, until they get available !
(Maybe even include a max delay between retries)

We require it for NATS sources, but the option could probably benefit from something similar for all network sources.

References

No response

Version

0.55

Start with the NATS source startup path and trace how unavailable sources currently prevent Vector from starting. Compare that behavior with the existing handling for a source that goes down after startup. Done means NATS sources can retry until available while Vector continues processing other configured sources, with any retry-delay behavior covered by tests.