Allow requiring OpenTelemetry protocol if needed

[twsouthwick]

For some scenarios (mine is running ASP.NET framework applications), gRPC is not supported. This change adds an overload for AddOtlpExporter APIs to have a required protocol. Since there's not a default http endpoint if one is not set, this will throw an exception pointing the user to set that variable.

Fixes #6789

[Copilot]

Pull Request Overview

This PR introduces the ability to explicitly require a specific OpenTelemetry protocol (gRPC or HTTP/Protobuf) when configuring the OTLP exporter, including new enums, overloads, annotations, and accompanying tests.

• Defines a new OtlpProtocol enum and extends the annotation model with RequiredProtocol
• Adds overloads for AddOtlpEnvironment and WithOtlpExporter to accept a protocol parameter
• Implements protocol-specific environment-variable registration logic and verifies behavior in tests

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/Aspire.Hosting.Tests/WithOtlpExporterTests.cs	Adds tests for protocol selection and exception when HTTP endpoint is missing
src/Aspire.Hosting/OtlpProtocol.cs	Introduces OtlpProtocol enum
src/Aspire.Hosting/OtlpConfigurationExtensions.cs	Implements protocol-aware environment setup and new overloads
src/Aspire.Hosting/ApplicationModel/OtlpExporterAnnotation.cs	Extends annotation model with nullable RequiredProtocol property

Comments suppressed due to low confidence (2)

src/Aspire.Hosting/OtlpConfigurationExtensions.cs:44

• The <param> description for protocol should be updated to reflect that this overload requires the protocol explicitly and does not use a fallback behavior. For example: "The protocol to use for the OTLP exporter."

    /// <param name="protocol">The protocol to use for the OTLP exporter. If not set, it will try gRPC then Http.</param>

src/Aspire.Hosting/ApplicationModel/OtlpExporterAnnotation.cs:15

• The summary refers to a "default protocol," but the property is RequiredProtocol. Consider updating to: "Gets or sets the required protocol for the OTLP exporter."

    /// Gets or sets the default protocol for the OTLP exporter.

[mitchdenny]

The existing AddOtlpEnvironment overload should probably call this one to reduce code duplication.

[twsouthwick]

The protocol parameter is non nullable here and so the RequiredProtocol is set to it, while if nothing is passed the RequiredProtocol will be null.

I did it this way since I don't think I can add a optional nullable parameter to the existing signature, to it was the simplest I could make it without having a third private method that took a nullable protocol which didn't seem to add much value here.