export logs over logs stream

[davidhewitt]

This is a follow-up to #56 to send all logs over a log exporter.

There are two reasons for this:

1. we support receiving logs in the logfire platform anyway, so makes sense to send this data over the corresponding stream
2. it means that users which send data to a separate sink which doesn't accept traces / spans can still see the logs

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.04535% with 88 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/internal/exporters/console.rs	83.83%	16 Missing and 11 partials ⚠️
src/exporters.rs	48.78%	19 Missing and 2 partials ⚠️
src/macros/impl_.rs	69.84%	17 Missing and 2 partials ⚠️
src/test_utils.rs	87.25%	4 Missing and 9 partials ⚠️
src/config.rs	75.00%	6 Missing ⚠️
src/lib.rs	95.23%	0 Missing and 2 partials ⚠️

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