kotel: optionally only use messaging.kafka.connects.count for connection metrics

[endorama]

This PR follows the discussion in https://github.com/twmb/franz-go/issues/670.

I added a new struct field, mergeConnectsMeter bool that is manipulated by WithMergedConnectsMeter(), to control whether to use one or 2 metrics to track connection successes/errors.

By default is set to false to retain current behaviour.

When set to true it disables the creation of messaging.kafka.connect_errors.count metric and follows a different path in Meter.OnBrokerConnect. There is some repetition as I wanted to avoid additional allocations when manipulating otel attributes (unfortunately Attribute Sets don't offer a way to add/remove elements from the set).

I added tests for the previous and new behaviour.

Let me know what you think, happy to adjust based on your feedback and thank you for your review!

[endorama]

@twmb I updated the PR to use attribute.Set and fixed the documentation. I also set this ready for review, happy to know what's your opinion!