feat(dashboard): Add ASP.NET Core Metrics dashboard (OTLP v1)

[manja316]

Summary

Adds a comprehensive ASP.NET Core monitoring dashboard for SigNoz using OTLP metrics.

• 20 panels across 4 sections: Application Performance, Request & Response, Process Metrics, Kestrel Server
• 2 template variables: service_name and deployment_environment
• Covers all metrics from the issue requirements: HTTP request rate, latency (P95), error rate, CPU, memory, GC, thread pool, exceptions, and Kestrel server metrics
• Follows SigNoz Query Builder format (no PromQL/ClickHouse SQL)
• Includes detailed README with setup instructions for OpenTelemetry .NET SDK

Metrics Used

Category	Metrics
HTTP Server	http.server.request.duration, http.server.active_requests
Process	process.cpu.time, process.memory.usage
.NET Runtime	process.runtime.dotnet.gc.collections.count, process.runtime.dotnet.gc.heap.size, process.runtime.dotnet.thread_pool.threads.count, process.runtime.dotnet.thread_pool.queue.length, process.runtime.dotnet.exceptions.count
Kestrel	kestrel.active_connections, kestrel.queued_connections, kestrel.request.duration

Closes SigNoz/signoz#5996

/claim #5996

[manja316]

Hi! Just following up on this ASP.NET Core monitoring dashboard PR. It covers all metrics from the issue requirements (HTTP rate, latency P95, CPU, memory, GC, thread pool, Kestrel). Let me know if there's anything I should adjust or if you'd prefer a different panel layout. Happy to iterate!

[manja316]

Gentle ping — this ASP.NET Core dashboard has been open 7+ days. It closes #7875 with full OTLP metrics coverage (HTTP, GC, thread pool, Kestrel). Happy to address any feedback!