Add local SLI observability stack and Grafana dashboard

Introduced a local observability stack using OpenTelemetry Collector, Prometheus, and Grafana with configuration and setup documentation. Added a prebuilt Grafana dashboard for SLI metrics, including panels for latency, success rate, error rates, and diagnostics. Enhanced the sample Web API with endpoints to emit various SLI outcomes for demonstration and testing. Updated documentation to guide users through setup and usage.

Author: xavierjohn

README.md

@@ -331,6 +331,8 @@ The default operation name is the HTTP method plus the route template (placehold
 
 Try out the sample weather forecast Web API.
 
+For a local Grafana/Prometheus/OpenTelemetry Collector experience, run the provisioned dashboard in [`sample\Observability\Grafana`](sample/Observability/Grafana/README.md). It shows SLI latency percentiles, success rate, failures, client errors, unknown customer diagnostics, and `<unrouted>` detection.
+
 To view the metrics locally using the [.NET Aspire Dashboard](https://aspire.dev/dashboard/standalone/):
 
 1. Start the Aspire dashboard:

sample/Observability/Grafana/README.md

@@ -0,0 +1,64 @@
+# Local SLI Grafana dashboard
+
+This sample runs a local OpenTelemetry Collector, Prometheus, and Grafana stack so you can see the value of the SLI library while running one of the sample applications.
+
+## Start the dashboard stack
+
+From this directory:
+
+```powershell
+docker compose up -d
+```
+
+Grafana starts at http://localhost:3000 with anonymous admin access enabled for local development. The SLI dashboard is provisioned automatically under **Dashboards > Trellis > Service Level Indicators**.
+
+![Trellis SLI Grafana dashboard](assets/sli-grafana-dashboard.png)
+
+## Run a sample app
+
+In another terminal, run the Web API sample and point its OTLP exporter at the local collector:
+
+```powershell
+$env:OTEL_EXPORTER_OTLP_ENDPOINT = "http://localhost:4317"
+dotnet run --project ..\..\WebApi\SampleWebApplicationSLI.csproj
+```
+
+Generate traffic:
+
+```powershell
+Invoke-RestMethod https://localhost:63936/WeatherForecast -SkipCertificateCheck
+Invoke-RestMethod https://localhost:63936/WeatherForecast/MyAction1 -SkipCertificateCheck
+Invoke-RestMethod https://localhost:63936/WeatherForecast/MyAction2 -SkipCertificateCheck
+Invoke-RestMethod https://localhost:63936/WeatherForecast/my-customer-resource-id -SkipCertificateCheck
+```
+
+You can also run the Minimal API or API-versioned samples with the same `OTEL_EXPORTER_OTLP_ENDPOINT` environment variable.
+
+## What the dashboard shows
+
+The dashboard uses the SLI metric contract:
+
+- `operation.duration` histogram exported to Prometheus as `operation_duration_milliseconds_*`
+- `CustomerResourceId`
+- `LocationId`
+- `Operation`
+- `Outcome`
+- `http.request.method`
+- `http.response.status.code`
+- optional `http.api.version`
+
+Panels include:
+
+- request volume by operation and outcome
+- p50/p95/p99 latency
+- success rate using `Success / (Success + Failure)`
+- failure and client-error rates
+- HTTP status-code breakdown
+- unknown customer diagnostics
+- `<unrouted>` operation detection
+
+## Stop the stack
+
+```powershell
+docker compose down
+```

sample/Observability/Grafana/assets/sli-grafana-dashboard.png

@@ -0,0 +1,44 @@
+services:
+  otel-collector:
+    image: otel/opentelemetry-collector-contrib:0.123.0
+    command: ["--config=/etc/otelcol/config.yaml"]
+    volumes:
+      - ./otel-collector-config.yaml:/etc/otelcol/config.yaml:ro
+    ports:
+      - "4317:4317"
+      - "4318:4318"
+      - "9464:9464"
+
+  prometheus:
+    image: prom/prometheus:v3.3.0
+    command:
+      - "--config.file=/etc/prometheus/prometheus.yml"
+      - "--storage.tsdb.path=/prometheus"
+      - "--web.enable-lifecycle"
+    volumes:
+      - ./prometheus.yml:/etc/prometheus/prometheus.yml:ro
+      - prometheus-data:/prometheus
+    ports:
+      - "9090:9090"
+    depends_on:
+      - otel-collector
+
+  grafana:
+    image: grafana/grafana:11.6.0
+    environment:
+      GF_AUTH_ANONYMOUS_ENABLED: "true"
+      GF_AUTH_ANONYMOUS_ORG_ROLE: "Admin"
+      GF_AUTH_DISABLE_LOGIN_FORM: "true"
+      GF_USERS_DEFAULT_THEME: "light"
+    volumes:
+      - grafana-data:/var/lib/grafana
+      - ./grafana/provisioning:/etc/grafana/provisioning:ro
+      - ./grafana/dashboards:/var/lib/grafana/dashboards:ro
+    ports:
+      - "3000:3000"
+    depends_on:
+      - prometheus
+
+volumes:
+  prometheus-data:
+  grafana-data: