Replace println calls with log calls

[0xd34d10cc]

This will allow users to format & redirect these messages via commonly used log libraries. minitrace-jaeger and minitrace-opentelemetry already had log dependency, so this was intended, but never actually implemented?

[coveralls]

Pull Request Test Coverage Report for Build 7731204918

• -1 of 3 (66.67%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 79.458%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
minitrace-jaeger/src/lib.rs	0	1	0.0%

Totals
Change from base Build 7664443160:	0.0%
Covered Lines:	1702
Relevant Lines:	2142

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💛 - Coveralls

[andylokandy]

Thank you!