Don't register a tracing dispatcher if no tracing env var was set.
Don't register a tracing dispatcher if no tracing env var was set.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust