Migrate otel metrics to use mdatagen: splunkhec exporter

[codeboten]

No description provided.

[github-actions]

Pinging code owners for exporter/splunkhec: @atoulme @dmitryax. See Adding Labels via Comments if you do not have permissions to add labels yourself.

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

Pinging code owners:

• exporter/splunkhec: @atoulme @dmitryax

See Adding Labels via Comments if you do not have permissions to add labels yourself.