[Hosting] Avoid allocations for OpenTelemetry

[martincostello]

Avoid allocations for OpenTelemetry

Reduce allocations for traces generated for HTTP requests.

Description

I was investigating why I wasn't getting the improvements I would expect with ASP.NET Core 11 preview.5 and OpenTelemetry.Instrumentation.AspNetCore 1.16.0 when the Microsoft.AspNetCore.Hosting.SuppressActivityOpenTelemetryData AppContext switch is set to false with Claude, which identified a number of possible ways to avoid allocations when OpenTelemetry is being generated.

The changes in this PR are essentially all the possible ways to avoid some of the allocations, whether they are worth doing and/or should be done in the ways suggested are for the team to decide.

The display name cache is probably the least controversial. The port boxing one could potentially be slimmed down to not care about 5000/5001/8080 and user-configured ports, and just keep the last-seen value for anything not 80 or 443, or just drop that part entirely.

Specific changes:

• Pre-box common/expected server.port values.
• Cache activity display names for endpoints.

I've proposed some similar changes for the OTel instrumentation for ASP.NET Core itself as well: open-telemetry/opentelemetry-dotnet-contrib#4600

[dotnet-policy-service]

Thanks for your PR, @martincostello. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Copilot]

Pull request overview

Reduces per-request allocations in the Hosting OpenTelemetry path by reusing boxed server.port values and caching Activity display names derived from HTTP method + route.

Changes:

• Reuse boxed instances for common ports and add a configurable pre-boxing mechanism for server-bound ports.
• Cache Activity.DisplayName strings for (method, route) to avoid per-request string construction.
• Register/unregister pre-boxed server ports during GenericWebHostService start/stop to avoid cross-host accumulation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Hosting/Hosting/src/Internal/HostingTelemetryHelpers.cs	Adds boxed-port reuse + registration APIs and introduces a display name cache for method/route pairs.
src/Hosting/Hosting/src/GenericHost/GenericWebHostService.cs	Collects bound/configured ports on startup and registers them for boxing reuse; unregisters on stop.