Improve OTLP observability example: add usings, reorder steps, refresh versions

[Copilot]

What was changed:

• Fixed the OTEL code so it now exports metrics to Aspire.
• Update packages from 1.9.0 (security vulnerability) to 1.17.0
• Described the namespaces a user needs.
• Describe the env vars that need to be set in appsettings.dev.json
• Let the user know that the CURL port is just an example and use port the website reports.
• Reordered the steps to prevent the user from ping-pong around the Program.cs file.
• Rewrite title/description for SEO.
• New first paragraph that describes what the user will learn by reading this.
• Edit pass.
• Moved source to its own folder per guidelines so changes don't impact other articles.

Fixes #45899
Fixes #55622

File	Preview link
docs/core/diagnostics/observability-otlp-example.md	Preview published page

Copilot's stuff...

The OTLP/Aspire Dashboard example article required trial and error to follow: it omitted using directives, had readers ping-pong between later and earlier insertion points in Program.cs, referenced an outdated package version, and hardcoded a port that doesn't match what dotnet run actually outputs.

Article structure

• Added a dedicated "Add using directives" step early in the walkthrough, backed by a new Snippet_Usings marker in Program.cs
• Reordered steps to follow the source file top-to-bottom: metrics/activity definitions and OTel provider configuration (both of which precede builder.Build();) now come before the API endpoint step (which comes after builder.Build();), so readers no longer jump backward to insert earlier code
• Updated internal step cross-references and renumbered the sub-headings (9.1/9.2/9.3) to match the new order

Content fixes

• Bumped the OpenTelemetry package version references from 1.9.0 to 1.17.0, matching the sample .csproj
• Replaced the hardcoded curl -k http://localhost:7275 example with guidance to read the actual listening port from the dotnet run console output, since it can differ per environment

// <Snippet_Usings>
using System.Diagnostics;
using System.Diagnostics.Metrics;
using OpenTelemetry.Metrics;
using OpenTelemetry.Resources;
using OpenTelemetry.Trace;
using OpenTelemetry.Exporter;
using OpenTelemetry.Logs;
using OpenTelemetry;
using Azure.Monitor.OpenTelemetry.AspNetCore;
// </Snippet_Usings>

• Fixes OTEL article could use more information to help coding #55622

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Internal previews

File	Preview link
docs/core/diagnostics/observability-otlp-example.md	Preview published page
docs/core/diagnostics/observability-with-otel.md	Preview published page
docs/core/diagnostics/snippets/observability-otlp-example/csharp/observability-otlp-example.csproj	Preview published page
docs/core/diagnostics/snippets/observability-otlp-example/csharp/Program.cs	Preview published page
docs/navigate/tools-diagnostics/toc.yml	Preview published page

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