Keep tokio console only in the playground

[squadgazzz]

Description

It turned out that tokio_unstable introduces a memory leak when creating spans. As @MartinquaXD noticed:

Most of the tracing related memory was allocated while holding a tokio mutex lock. When you check the tokio code you see that this only happens when we build it with tokio_unstable which is only used for tokio-console support.

Disabling the tokio_unstable helped to mitigate the memory leak issue almost completely.

Changes

• Remove tokio_unstable from the codebase.
• Keep it only in the playground (for now).

Further implementation

Using the updated deployment CI job from #3954, the tokio-console can be gated behind a feature in case we need to run it in the future. Will implement it in a follow-up pr.

[Copilot]

Pull request overview

This PR addresses a memory leak issue caused by tokio_unstable by removing it from the global build configuration and restricting tokio-console support to the playground environment only. The changes introduce a feature flag approach that allows tokio-console to be enabled on-demand rather than globally.

• Removed global tokio_unstable configuration from .cargo/config.toml
• Converted console-subscriber to an optional dependency gated behind a tokio-console feature flag
• Updated playground Dockerfile to explicitly enable tokio-console feature for all packages

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
.cargo/config.toml	Removed global tokio_unstable rustflag configuration to prevent memory leaks in production
crates/observe/Cargo.toml	Made console-subscriber optional and added tokio-console feature flag
crates/observe/build.rs	Added clarifying comments about tokio_unstable usage in playground
crates/observe/src/tracing.rs	Refactored tokio-console initialization to use conditional compilation with both tokio_unstable cfg and tokio-console feature flag
playground/Dockerfile	Added --features observe/tokio-console to all cargo build commands to enable tokio-console in playground environment

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[Copilot]

The .unwrap() call could panic if the TOKIO_CONSOLE environment variable is set to an invalid value (e.g., "maybe", "1", "yes"). Consider using .unwrap_or(false) or .unwrap_or_else(|_| false) to handle parsing errors gracefully, defaulting to false when the value cannot be parsed as a boolean.

Suggested change

	.unwrap();
	.unwrap_or(false);