Initial release of payloadmeter

feat(metering): add metering for logs (#493)

#### Features

- Add a pkg `metering` with interface `metering.Meter` for Size and Count usage calculation
- Add a pkg `pdatagen` for generating a wide variety of telemetry data for testing

Add missing string value length limits

Adding meter package

Adding payloadmeter go module

Adding payloadmeter go module

Adding payloadmeter go module

Initial release of payloadmeter

Initial release of payloadmeter

fix: nil meter instance used in exporter (#492)

Fixes #491