Default ping-mode to ping-only for now

[moskyb]

Description

In #3697, we introduced a new feature to the agent to stream ping messages from the buildkite backend, rather than polling. For long-running agents, this should greatly decrease the amount of time between a job being assigned and the agent picking it up.

In this PR, we added a config item called --ping-mode, which allows users to pick which ping mode they want to use, and defaulted it to auto. When ping-mode is auto, the agent will attempt to use streaming pings, but if they're unavailable, it will fall back to polling.

Unfortunately, because this feature is still under construction on the backend, at the time of writing, all attempts to connect to a ping stream will fail, outputting some error logs into the agent logs:

Because of the semantics of --ping-mode auto, the agent immediately falls back to using the ping handler, so there's very little actual impact from this issue. It does look like a problem though, so this PR sets the default --ping-mode to ping-only for now. In a future release, we'll set the default back to auto.

Context

Slack convo

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

n/a