[Azure Functions] Add architecture deep dive documentation (#7639)

## Summary of changes

Adds comprehensive architectural documentation for Azure Functions Host
and .NET Worker relevant to `dd-trace-dotnet` instrumentation.

**New**: `docs/development/AzureFunctions-Architecture.md` (922 lines)
- Host and Worker architecture, components, and execution pipeline
- gRPC communication protocol and message types
- Distributed tracing integration (W3C TraceContext, Activity,
OpenTelemetry)
- Environment variables, configuration, and instrumentation hook points

**Updated**: `AGENTS.md` and `docs/development/AzureFunctions.md` with
cross-references

## Reason for change

Makes it easier to understand Azure Functions internals, navigate source
code, identify instrumentation points, and debug issues.

## Implementation details

Created by analyzing the [Azure Functions
Host](https://github.com/Azure/azure-functions-host) and [.NET
Worker](https://github.com/Azure/azure-functions-dotnet-worker)
repositories. Includes specific file paths and line numbers for key
components.

## Test coverage

Documentation only - no code changes.

## Other details

Internal developer documentation complementing the existing
`AzureFunctions.md` integration guide.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude <[email protected]>

Author: lucaspimentel

AGENTS.md

@@ -19,6 +19,25 @@
 - Loader/home: Build outputs publish a "monitoring home"; the native loader boots the tracer from there.
 - Build system: Nuke coordinates .NET builds and CMake/vcpkg for native components.
 
+## NuGet Package Architecture
+
+### Datadog.Trace Package
+The `Datadog.Trace` NuGet package provides the **manual instrumentation API** for customers:
+- **Contains**: `Datadog.Trace.Manual.dll` - Public API for manual instrumentation
+- **Does NOT contain**: Auto-instrumentation code or native profiler binaries
+- **Usage**: Reference in application code for manual tracing (e.g., `Tracer.Instance.StartActive()`)
+
+Auto-instrumentation comes from the tracer "monitoring home" deployed separately (via installers, MSI, container images, or specialized packages like `Datadog.AzureFunctions`).
+
+### Datadog.Trace.dll vs Datadog.Trace.Manual.dll
+- `Datadog.Trace.dll` - The full managed tracer with all auto-instrumentation code, loaded by the native profiler into instrumented processes
+- `Datadog.Trace.Manual.dll` - Lightweight manual instrumentation API packaged in the `Datadog.Trace` NuGet package for customer reference
+
+### Specialized Packages
+- **Datadog.AzureFunctions**: Bundles `Datadog.Trace.dll` and native profiler for Azure Functions (see `docs/development/AzureFunctions.md`)
+- **Datadog.Monitoring.Distribution**: MSI installer for Windows (IIS, Windows Services)
+- Other serverless/platform-specific packages may bundle the full tracer similarly
+
 ## Tracer Structure
 
 - `tracer/src/Datadog.Trace` — Core managed tracer library
@@ -199,10 +218,14 @@ tracer/src/Datadog.Trace
 - **Setup**: Use Azure App Services Site Extension on Windows Premium/Elastic Premium/Dedicated plans; use `Datadog.AzureFunctions` NuGet package for Linux Consumption/Container Apps
 - **Tests**: `BuildAndRunWindowsAzureFunctionsTests` Nuke target; samples under `tracer/test/test-applications/azure-functions/`
 - **Dependencies**: `Datadog.AzureFunctions` → `Datadog.Serverless.Compat` ([datadog-serverless-compat-dotnet](https://github.com/DataDog/datadog-serverless-compat-dotnet)) contains agent executable
+- **External Repos**: [Azure Functions Host](https://github.com/Azure/azure-functions-host) and [.NET Worker](https://github.com/Azure/azure-functions-dotnet-worker)
 
 📖 **Load when**: Working on Azure Functions instrumentation or debugging serverless issues
 - **`docs/development/AzureFunctions.md`** — In-process vs isolated worker models, instrumentation specifics, ASP.NET Core integration, GRPC context propagation, and debugging guide
 
+📖 **Load when**: Need detailed architectural understanding of Azure Functions internals
+- **`docs/development/AzureFunctions-Architecture.md`** — Deep dive into Azure Functions Host and .NET Worker architecture, gRPC protocol, middleware model, distributed tracing integration, environment variables, and instrumentation hook points
+
 📖 **Load when**: Working on AWS Lambda or general serverless instrumentation
 - **`docs/development/Serverless.md`** — Serverless instrumentation patterns across cloud providers
 
@@ -292,8 +315,10 @@ The tracer runs in-process with customer applications and must have minimal perf
 - `docs/development/AutomaticInstrumentation.md` — Creating integrations
 - `docs/development/DuckTyping.md` — Duck typing guide
 - `docs/development/AzureFunctions.md` — Azure Functions integration
+- `docs/development/AzureFunctions-Architecture.md` — Azure Functions architecture deep dive
 - `docs/development/Serverless.md` — Serverless instrumentation
 - `docs/development/UpdatingTheSdk.md` — SDK updates
+- `docs/development/QueryingDatadogAPIs.md` — Querying Datadog APIs for debugging (spans, logs)
 
 **CI & Testing:**
 - `docs/development/CI/TroubleshootingCIFailures.md` — Investigating build/test failures in Azure DevOps