Add Prometheus metrics endpoint; containerize integration tests

Prometheus metrics (internal/metrics/, cmd/maglevd/)
- New --metrics-addr flag (default :9091, env MAGLEV_METRICS_ADDR)
  serving /metrics via promhttp.
- Gauge metrics scraped on demand via a custom prometheus.Collector:
  maglev_backend_state, maglev_backend_health, maglev_backend_enabled,
  maglev_frontend_pool_backend_weight.
- Inline counter/histogram metrics updated per probe:
  maglev_probe_total (by backend, type, result, code),
  maglev_probe_duration_seconds (by backend, type),
  maglev_backend_transitions_total (by backend, from, to).
- StateSource interface in metrics package breaks the import cycle
  with checker; checker.Checker satisfies it via GetBackendInfo.

Integration tests
- Run maglevd inside a containerlab node (debian:trixie-slim with
  build/ bind-mounted) instead of on the host. Eliminates port
  collisions with any host maglevd.
- maglevc commands run via docker exec into the maglevd container.
- Add 6 Prometheus test cases: endpoint reachable, all backends
  report state=up, probe counters non-zero, duration histogram
  populated, pool weights correct, transition counters present.

tests/01-maglevd/01-healthcheck.robot

@@ -1,6 +1,5 @@
 *** Settings ***
 Library             OperatingSystem
-Library             Process
 Resource            ../common.robot
 
 Suite Setup         Setup Suite
@@ -10,26 +9,20 @@ Suite Teardown      Cleanup Suite
 *** Variables ***
 ${lab-name}         maglevd-test
 ${lab-file}         maglevd-lab/maglevd.clab.yml
-${config-file}      maglevd-lab/maglev.yaml
 ${runtime}          docker
-${GRPC_PORT}        9091
+${MAGLEVD_NODE}     clab-maglevd-test-maglevd
+${METRICS_URL}      http://172.20.30.2:9091/metrics
 
 
 *** Test Cases ***
 Deploy maglevd-test lab
+    [Documentation]    Deploy the containerlab topology. The maglevd node starts
+    ...    automatically as PID 1 via start.sh and begins probing the nginx
+    ...    backends immediately.
     ${rc}    ${output} =    Run And Return Rc And Output
     ...    ${CLAB_BIN} --runtime ${runtime} deploy -t ${CURDIR}/${lab-file}
     Log    ${output}
     Should Be Equal As Integers    ${rc}    0
-
-Start maglevd
-    ${handle} =    Start Process    ${MAGLEVD}
-    ...    --config    ${CURDIR}/${config-file}
-    ...    --grpc-addr    :${GRPC_PORT}
-    ...    --log-level    debug
-    ...    alias=maglevd    stdout=${EXECDIR}/tests/out/maglevd.log
-    ...    stderr=STDOUT
-    Set Suite Variable    ${MAGLEVD_HANDLE}    ${handle}
     Sleep    3s    Wait for nginx containers and probes to converge
 
 All backends reach up state
@@ -86,22 +79,55 @@ Enable backend restarts probing
     ...    Backend Should Be Up    nginx2
 
 
+Prometheus endpoint is reachable
+    ${rc}    ${output} =    Run And Return Rc And Output
+    ...    curl -sf ${METRICS_URL}
+    Log    ${output}
+    Should Be Equal As Integers    ${rc}    0
+    Should Contain    ${output}    maglev_backend_state
+
+Prometheus reports all backends up
+    ${output} =    Scrape Metrics
+    # Each backend should have state="up" = 1.
+    Should Contain    ${output}    maglev_backend_state{address="172.20.30.11",backend="nginx1",healthcheck="http-check",state="up"} 1
+    Should Contain    ${output}    maglev_backend_state{address="172.20.30.12",backend="nginx2",healthcheck="http-check",state="up"} 1
+    Should Contain    ${output}    maglev_backend_state{address="172.20.30.13",backend="nginx3",healthcheck="http-check",state="up"} 1
+
+Prometheus reports probe counters
+    ${output} =    Scrape Metrics
+    Should Match Regexp    ${output}    maglev_probe_total\\{backend="nginx1".*result="success".*\\}\\s+[1-9]
+    Should Match Regexp    ${output}    maglev_probe_total\\{backend="nginx2".*result="success".*\\}\\s+[1-9]
+    Should Match Regexp    ${output}    maglev_probe_total\\{backend="nginx3".*result="success".*\\}\\s+[1-9]
+
+Prometheus reports probe duration histogram
+    ${output} =    Scrape Metrics
+    Should Match Regexp    ${output}    maglev_probe_duration_seconds_count\\{backend="nginx1".*\\}\\s+[1-9]
+
+Prometheus reports pool weights
+    ${output} =    Scrape Metrics
+    Should Contain    ${output}    maglev_frontend_pool_backend_weight{backend="nginx1",frontend="http-vip",pool="primary"} 100
+    Should Contain    ${output}    maglev_frontend_pool_backend_weight{backend="nginx3",frontend="http-vip",pool="fallback"} 100
+
+Prometheus reports transition counters
+    ${output} =    Scrape Metrics
+    # All backends transitioned unknown → up during startup.
+    Should Match Regexp    ${output}    maglev_backend_transitions_total\\{backend="nginx1",from="unknown",to="up"\\}\\s+[1-9]
+
+
 *** Keywords ***
 Setup Suite
     ${arch} =    Run    go env GOARCH
     Set Suite Variable    ${ARCH}    ${arch}
-    Set Suite Variable    ${MAGLEVD}    ${EXECDIR}/build/${ARCH}/maglevd
-    Set Suite Variable    ${MAGLEVC}    ${EXECDIR}/build/${ARCH}/maglevc
 
 Cleanup Suite
-    Run Keyword And Ignore Error    Terminate Process    maglevd    kill=true
+    Run    docker logs ${MAGLEVD_NODE} > ${EXECDIR}/tests/out/maglevd.log 2>&1
     Run    ${CLAB_BIN} --runtime ${runtime} destroy -t ${CURDIR}/${lab-file} --cleanup
 
 Maglevc
-    [Documentation]    Run a maglevc command and return its output.
+    [Documentation]    Run a maglevc command inside the maglevd container.
     [Arguments]    ${cmd}
     ${rc}    ${output} =    Run And Return Rc And Output
-    ...    ${MAGLEVC} --server\=localhost:${GRPC_PORT} --color\=false ${cmd}
+    ...    docker exec ${MAGLEVD_NODE} /opt/maglev/build/${ARCH}/maglevc --color\=false ${cmd}
     Log    ${output}
     Should Be Equal As Integers    ${rc}    0
     RETURN    ${output}
@@ -133,3 +159,10 @@ Probe Count Should Be Positive
     ${count} =    Get Probe Count    ${name}
     Should Be True    ${count} > 0
     ...    No health-check requests found in nginx logs for ${name}
+
+Scrape Metrics
+    [Documentation]    Fetch the Prometheus /metrics endpoint from the maglevd container.
+    ${rc}    ${output} =    Run And Return Rc And Output
+    ...    curl -sf ${METRICS_URL}
+    Should Be Equal As Integers    ${rc}    0
+    RETURN    ${output}

tests/01-maglevd/maglevd-lab/maglevd.clab.yml

@@ -6,6 +6,15 @@ mgmt:
 
 topology:
   nodes:
+    maglevd:
+      kind: linux
+      image: debian:trixie-slim
+      mgmt-ipv4: 172.20.30.2
+      binds:
+        - ../../../build:/opt/maglev/build:ro
+        - ./maglev.yaml:/etc/maglev/maglev.yaml:ro
+        - ./start.sh:/start.sh:ro
+      cmd: /start.sh
     nginx1:
       kind: linux
       image: nginx:alpine

tests/01-maglevd/maglevd-lab/start.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+ARCH=$(uname -m)
+case "$ARCH" in
+    x86_64)  ARCH=amd64 ;;
+    aarch64) ARCH=arm64 ;;
+esac
+exec /opt/maglev/build/${ARCH}/maglevd --config /etc/maglev/maglev.yaml --log-level debug