Update example custom collector to inherit from BaseCollector

[geoffharcourt]

The documentation advises that a custom collector should inherit from PrometheusExporter::Server::BaseCollector and the example in the docs does so. The code example inherits from Collector instead. We found this seemed to work in development but would behave unreliably in production. This change updates the example to match the code snippet in the Readme.

[SamSaffron]

Looks good, thanks for that!