feat(bff): expose OTel golden-signal metrics on /metrics (refs #124)

services/bff/Bff.Api/Bff.Api.csproj

@@ -11,6 +11,7 @@
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="10.0.8" />
     <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="10.0.8" />
     <PackageReference Include="OpenTelemetry.Exporter.OpenTelemetryProtocol" Version="1.17.0" />
+    <PackageReference Include="OpenTelemetry.Exporter.Prometheus.AspNetCore" Version="1.17.0-beta.1" />
     <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.17.0" />
     <PackageReference Include="OpenTelemetry.Instrumentation.AspNetCore" Version="1.17.0" />
     <PackageReference Include="OpenTelemetry.Instrumentation.Http" Version="1.17.0" />

services/bff/Bff.Api/Program.cs

@@ -3,6 +3,7 @@ using System.Text.Json;
 using System.Text.Json.Serialization;
 using Bff.Api;
 using Microsoft.AspNetCore.Authentication.JwtBearer;
+using OpenTelemetry.Metrics;
 using OpenTelemetry.Resources;
 using OpenTelemetry.Trace;
 
@@ -18,7 +19,16 @@ builder.Services.AddOpenTelemetry()
     .WithTracing(tracing => tracing
         .AddAspNetCoreInstrumentation(o => o.Filter = ctx => ctx.Request.Path != "/health")
         .AddHttpClientInstrumentation()
-        .AddOtlpExporter());
+        .AddOtlpExporter())
+    // OpenTelemetry metrics (S-16c, ADR-0023): the golden signals for the request path —
+    // http.server.request.duration (traffic/errors/latency) + http.client.* for the downstream hops,
+    // plus the built-in System.Runtime meter for saturation (GC, CPU, thread pool). Prometheus scrapes
+    // these from /metrics (mapped below); no OTLP push for metrics, so no collector hop (ADR-0023).
+    .WithMetrics(metrics => metrics
+        .AddAspNetCoreInstrumentation()
+        .AddHttpClientInstrumentation()
+        .AddMeter("System.Runtime")
+        .AddPrometheusExporter());
 
 var keycloakAuthority = builder.Configuration["Keycloak:Authority"]
     ?? throw new InvalidOperationException("Missing configuration 'Keycloak:Authority'");
@@ -82,6 +92,9 @@ app.UseAuthentication();
 app.UseAuthorization();
 
 app.MapHealthChecks("/health");
+
+// Prometheus scrape endpoint (S-16c): exposes the OTel metrics above in Prometheus text format.
+app.MapPrometheusScrapingEndpoint();
 app.MapOpenApi();
 
 // Self-service submit: requires a valid digid token; the bsn comes from the token, not the body,