Consider enabling exemplars in template service defaults

### Is there an existing issue for this?

- [X] I have searched the existing issues

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

Metrics exemplars are off by default. `opentelemetry-dotnet` decided to do this because there is a small perf cost to collecting them.

In Aspire, exemplars are enabled locally via an env var that is set on .NET projects. However, this won't be applied after deployment.

### Describe the solution you'd like

Enable exemplars in service defaults template. This would be some configuration of `opentelemetry-dotnet`.

### Additional context

_No response_

Locate the service defaults template and its OpenTelemetry configuration, then read how the template sets local versus deployed environment defaults. Enable metrics exemplars in that configuration and verify that the generated service defaults apply them after deployment.

Written by the indexing model from the issue text.