Use defaults for AzureMonitorExporterOptions to allow configuration of sampling etc

[agehrke]

Issue describing the changes in this PR

Currently when configuring the function host to use OpenTelemetry it adds Azure Monitor Exporter through UseAzureMonitorExporter(this IOpenTelemetryBuilder builder, Action<AzureMonitorExporterOptions> configureAzureMonitor) extension method. This unfortunately bypasses the nice defaults in DefaultAzureMonitorExporterOptions which configures sampling via the OTEL environment variables OTEL_TRACES_SAMPLER and OTEL_TRACES_SAMPLER_ARG.

Should resolve #11496.

Pull request checklist

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