add must_use for CollectorBuilder::finish

[kartva]

Motivation

Follow up to #2239 Discovered that

    tracing_subscriber::fmt().with_ansi(false).finish();
    tracing::info!("hello");

doesn't actually do anything, because finish returns a Collector and init is the thing that actually installs the collector.

Solution

Added #[must_use] which suggests using try_init instead.