Discover the agent address from BLACKSMITH_AGENT_ADDR

[piob-io]

Discovers the Blacksmith agent through the BLACKSMITH_AGENT_ADDR env var the agent advertises on every platform (FastActions/fa#4816); the agent gRPC client, the internal metrics endpoint, and the Docker Hub mirror in the buildkitd registry config all derive from it, with no hardcoded agent IPs. Agent unavailability never fails the job:

• BLACKSMITH_AGENT_ADDR unset (GitHub-hosted runner, or an agent that does not export it): warn, fall back to the local docker-container builder, omit the registry mirror config, skip metrics reporting, zero agent dials.
• Agent answers Unimplemented (service not offered on this platform): same local-builder fallback.
• Dial refused or timed out while BLACKSMITH_AGENT_ADDR is set: same local-builder fallback; the failure is still reported to the backend so agent outages stay visible.
• nofallback: true fails on any error.

Only genuine agent failures reach /stickydisks/report-failed; environments without agent support are classified as "unsupported" and skipped, so non-Blacksmith and non-Linux runs do not inflate the docker setup failure metrics:

if (!reporter.isAgentUnsupportedError(error)) {
  await reporter.reportBuildPushActionFailure("STICKYDISK_SETUP", error, "sticky disk setup");
}

Requires the agent change (FastActions/fa#4816) to be rolled out fleet-wide before this is released.

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[taha-au]

I'd have expected the presence/absence of BLACKSMITH_STICKY_DISK_GRPC_PORT to be authoritative for sticky disk support, rather than BLACKSMITH_AGENT_ADDR. the exposition and use of BLACKSMITH_AGENT_ADDR itself for discovering agent address makes sense though

[piob-io]

I'd have expected the presence/absence of BLACKSMITH_STICKY_DISK_GRPC_PORT to be authoritative for sticky disk support, rather than BLACKSMITH_AGENT_ADDR. the exposition and use of BLACKSMITH_AGENT_ADDR itself for discovering agent address makes sense though

This was my original plan. However, for old action versions failing well, I needed to have the agent always expose the stickydisk endpoints, and just respond with unsupported error. This approach will keep these endpoints excercised, so we don't risk breaking this endpoints, and not noticing that we degraded for some old version users (https://github.com/FastActions/fa/pull/4816).

I will cleanup the BLACKSMITH_STICKY_DISK_GRPC_PORT fallback value tho.