Fix BatchExporter to match documentation

The BatchExporter starts automatically now to match documentation

v0.3.2

Just like in v0.3.1, a runtime dependency was accidentally set as development instead

v0.3.1

• Fixed a dependency that was accidentally specified as a development dependency

v0.3.0

• Fixed version mismatch between git tag and shard.yml

v0.2.0

• Added NullExporter as the default so that exporting traces won't crash if OpenTelemetry.configure hasn't been called yet
• Added support for all Crystal 1.x releases to shard.yml