OTel Prometheus exporter wiring — RedundancyMetrics meter now scraped at /metrics. Closes task #201. Picked Prometheus over OTLP per the earlier recommendation (pull-based means no OTel Collector deployment required for the common K8s/containers case; the endpoint ASP.NET-hosts inside the Admin app already, so one less moving part). Adds two NuGet refs to the Admin csproj: OpenTelemetry.Extensions.Hosting 1.15.2 (stable) + OpenTelemetry.Exporter.Prometheus.AspNetCore 1.15.2-beta.1 (the exporter has historically been beta-only; rest of the OTel ecosystem treats it as production-acceptable + it's what the upstream OTel docs themselves recommend for AspNetCore hosts). Program.cs gains a Metrics:Prometheus:Enabled toggle (defaults true; setting to false disables both the MeterProvider registration + the scrape endpoint entirely for locked-down deployments). When enabled, AddOpenTelemetry().WithMetrics() registers a MeterProvider that subscribes to the "ZB.MOM.WW.OtOpcUa.Redundancy" meter (the exact MeterName constant on RedundancyMetrics) + wires AddPrometheusExporter. MapPrometheusScrapingEndpoint() appends a /metrics handler producing the Prometheus text-format output; deliberately NOT authenticated because scrape jobs typically run on a trusted network + operators who need auth wrap the endpoint behind a reverse-proxy basic-auth gate per fleet-ops convention. appsettings.json declares the toggle with Enabled: true so the default deploy gets metrics automatically — turning off is the explicit action. Future meters (resilience tracker + host status + auth probe) just AddMeter("Name") alongside the existing call to start flowing through the same endpoint without more infrastructure. Admin project builds 0 errors; Admin.Tests 92/92 passing (unchanged — the OTel pipeline runs at request time, not test time). Still-pending work that was NOT part of #201's scope: an equivalent setup for the Server project (different MeterNames — the Polly pipeline builder's tracker + host-status publisher) + a metrics cheat-sheet in docs/observability.md documenting each meter's tag set + expected alerting thresholds. Those are natural follow-ups when fleet-ops starts building dashboards.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Admin/Program.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Authentication.Cookies;
 using Microsoft.EntityFrameworkCore;
+using OpenTelemetry.Metrics;
 using Serilog;
 using ZB.MOM.WW.OtOpcUa.Admin.Components;
 using ZB.MOM.WW.OtOpcUa.Admin.Hubs;
@@ -70,6 +71,19 @@ builder.Services.AddScoped<ILdapAuthService, LdapAuthService>();
 // SignalR real-time fleet status + alerts (admin-ui.md §"Real-Time Updates").
 builder.Services.AddHostedService<FleetStatusPoller>();
 
+// OpenTelemetry Prometheus exporter — Meter stream from RedundancyMetrics + any future
+// Admin-side instrumentation lands on the /metrics endpoint Prometheus scrapes. Pull-based
+// means no OTel Collector deployment required for the common deploy-in-a-K8s case; appsettings
+// Metrics:Prometheus:Enabled=false disables the endpoint entirely for locked-down deployments.
+var metricsEnabled = builder.Configuration.GetValue("Metrics:Prometheus:Enabled", true);
+if (metricsEnabled)
+{
+    builder.Services.AddOpenTelemetry()
+        .WithMetrics(m => m
+            .AddMeter(RedundancyMetrics.MeterName)
+            .AddPrometheusExporter());
+}
+
 var app = builder.Build();
 
 app.UseSerilogRequestLogging();
@@ -87,6 +101,15 @@ app.MapPost("/auth/logout", async (HttpContext ctx) =>
 app.MapHub<FleetStatusHub>("/hubs/fleet");
 app.MapHub<AlertHub>("/hubs/alerts");
 
+if (metricsEnabled)
+{
+    // Prometheus scrape endpoint — expose instrumentation registered in the OTel MeterProvider
+    // above. Emits text-format metrics at /metrics; auth is intentionally NOT required (Prometheus
+    // scrape jobs typically run on a trusted network). Operators who need auth put the endpoint
+    // behind a reverse-proxy basic-auth gate per fleet-ops convention.
+    app.MapPrometheusScrapingEndpoint();
+}
+
 app.MapRazorComponents<App>().AddInteractiveServerRenderMode();
 
 await app.RunAsync();

src/ZB.MOM.WW.OtOpcUa.Admin/ZB.MOM.WW.OtOpcUa.Admin.csproj

@@ -16,6 +16,8 @@
         <PackageReference Include="Novell.Directory.Ldap.NETStandard" Version="3.6.0"/>
         <PackageReference Include="Microsoft.AspNetCore.SignalR.Client" Version="10.0.0"/>
         <PackageReference Include="Serilog.AspNetCore" Version="9.0.0"/>
+        <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.15.2"/>
+        <PackageReference Include="OpenTelemetry.Exporter.Prometheus.AspNetCore" Version="1.15.2-beta.1"/>
     </ItemGroup>
 
     <ItemGroup>

src/ZB.MOM.WW.OtOpcUa.Admin/appsettings.json

@@ -23,5 +23,10 @@
   },
   "Serilog": {
     "MinimumLevel": "Information"
+  },
+  "Metrics": {
+    "Prometheus": {
+      "Enabled": true
+    }
   }
 }