Fix Metrics export in C# sample.

Currently metrics are not being exported from C# sample to GMP. The debug exporter confirms OTLP metrics being produced, but they fail to export with:

`{
"textPayload": "2025-08-18T18:40:38.328Z\terror\tinternal/queue_sender.go:57\tExporting failed. Dropping data.\t{\"resource\": {\"service.instance.id\": \"049e2d35-9e72-4142-9dd4-95a3d227a585\", \"service.name\": \"otelcol-google\", \"service.version\": \"0.128.0\"}, \"otelcol.component.id\": \"googlemanagedprometheus\", \"otelcol.component.kind\": \"exporter\", \"otelcol.signal\": \"metrics\", \"error\": \"rpc error: code = Internal desc = Internal error encountered. Please retry after a few seconds. If internal errors persist, contact support at https://cloud.google.com/support/docs.\", \"dropped_items\": 46}",
}`

Start with the C# sample's OTLP metrics export path and reproduce the failure while comparing it with the debug exporter output. Use the Google Managed Prometheus exporter error and dropped-items log to trace where export fails. Done means the sample's metrics reach GMP without being dropped.

Written by the indexing model from the issue text.