fix(stackable-telemetry): Disable ANSI colors when stdout is not a terminal

[lfrancke]

Summary

• Only emit ANSI escape codes in console log output when stdout is actually a terminal
• Applies to both Plain and Json console log formats

Problem

tracing_subscriber::fmt::layer() enables ANSI colors by default whenever the ansi feature is enabled and NO_COLOR is unset (source):

let ansi = cfg!(feature = "ansi") && env::var("NO_COLOR").map_or(true, |v| v.is_empty());

There is no automatic TTY detection — this is a known upstream issue open since 2021. As a result, containerdebug (and any other consumer of stackable-telemetry) emits raw ANSI codes when running in containers where logs are collected by aggregation systems like Graylog or Vector:

[2m2026-03-29T18:26:23.658052Z[0m [32m INFO[0m [1mcontainerdebug run[0m...

Fix

Explicitly check std::io::stdout().is_terminal() before configuring the subscriber, so ANSI is only used when writing to an actual terminal.

Test plan

• cargo check -p stackable-telemetry passes
• Verify in a terminal: colored output still works
• Verify in a container/pipe: ANSI codes are suppressed

[NickLarsenNZ]

LGTM

[Techassi]

This should include a changelog entry. Otherwise looks good.

[NickLarsenNZ]

I will try to get around to testing this with containerdebug.

[Techassi]

Okay I have confirmed that both

cargo run -- --output state.json > logs.txt
cargo run -- --output state.json

produce the expected output:

• When logging to a terminal, the ANSI escape sequences are emitted which results in colored console output.
• When piped to a text file, the log messages are written as plain text.

This PR can be merged and released which allows us to bump to a never stackable-telemetry version in containerdebug.