Update templates' service defaults to enable metric exemplars

[JamesNK]

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Is your feature request related to a problem? Please describe the problem.

PR #4629 updates opentelemetry-dotnet to a version that supports metrics exemplars. It sets environment variables to enable metric exemplars in a dev environment.

Consider updating Aspire service defaults to enable metric exemplars so they're enabled after deployment.

Describe the solution you'd like

Update service defaults to enable metric exemplars.

Additional context

No response

[DamianEdwards]

Seems reasonable. Would accept a PR for this one but not important enough to put it into a milestone for now IMO.

[DamianEdwards]

The linked PR did not update the templates, just the playground apps.