fix prometheus log message terminology

[gangelop]

A prometheus "server" typically refers to the prometheus process running
on a central server which connects to various prometheus "exporters" and
collects metrics. What is implemented here in substrate is a prometheus
exporter.

This patch fixes the associated log message to avoid confusion for new
users.

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[gangelop]

Forgot to mention this for reference:
https://prometheus.io/docs/introduction/glossary/#exporter

[asymmetric]

Thank you for this one, was just scratching my head about what the log message actually meant!