static-exporter: Implement exporter using busybox (#1521)

* static-exporter: Implement exporter using busybox

 Add shell-based implementation for static-exporter using busybox as a lightweight alternative to the Apache web server implementation.

  The new implementation addresses dependency management concerns by providing a distroless container option that runs as non-root and includes built-in health checks.

  Changes

  - Added shell-based exporter option: New shell_exporter=true parameter enables busybox implementation
  - Automatic image selection: Uses gcr.io/distroless/static-debian12:debug for shell mode vs httpd:2.4-alpine for Apache
  - Embedded HTTP server: Pure bash implementation that serves /metrics and /health endpoints using netcat
  - Security improvements: Non-root execution (uid/gid 65532) with proper security context
  - Health checks: Added readiness probe for the shell implementation
  - Updated documentation: README now explains both implementations and their use cases

* Update static-exporter/main.libsonnet

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update static-exporter/README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Refactor into own function

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: simonswine

static-exporter/README.md

@@ -44,6 +44,19 @@ local static_exporter = import 'github.com/grafana/jsonnet-libs/static-expoter/m
 }
 ```
 
+## Using shell based implementation
+
+The original implementation using the Apache web server contains quite a few dependencies that might be tricky to keep updated. An optional [busybox] based implementation can be used by using the `newShellExporter` function:
+
+```jsonnet
+    static_exporter.newShellExporter('team-holiday-exporter')
+```
+
+This variant uses a [distroless] image, runs as non-root and comes with a health check.
+
+[distroless]:https://github.com/GoogleContainerTools/distroless
+[busybox]:https://busybox.net/
+
 ## Updating httpd.conf
 
 There is a default httpd.conf that was added to this library.
@@ -61,4 +74,4 @@ If there is a downstream change that requires updating this config file, run the
     </IfModule>
 ```
 
-This change adds a Header to the requests that enables Prometheus 3.x to scrape the static exporter.
+This change adds a Header to the requests that enables Prometheus 3.x to scrape the static exporter.

static-exporter/main.libsonnet

@@ -28,6 +28,100 @@ local k = import 'ksonnet-util/kausal.libsonnet';
     + self.withHttpConfig()
   ,
 
+  newShellExporter(name, image='gcr.io/distroless/static-debian12:debug', port=8080)::
+    {
+      name:: name,
+      data:: {
+        metrics: '',
+        handler: |||
+          METRICS_FILE="/data/metrics"
+
+          handle_request() {
+              local request_line
+              read -r request_line
+
+              # Parse HTTP method and path
+              local method=$(echo "$request_line" | cut -d' ' -f1)
+              local path=$(echo "$request_line" | cut -d' ' -f2)
+
+              # Read and discard headers
+              while IFS= read -r line && [ "$line" != $'\r' ]; do
+                  :
+              done
+
+              if [[ "$path" == "/metrics" ]]; then
+                  # Serve Prometheus metrics
+                  echo "HTTP/1.1 200 OK"
+                  echo "Connection: close"
+                  echo "Content-Type: text/plain; version=0.0.4; charset=utf-8"
+                  echo "Content-Length: $(wc -c < "$METRICS_FILE")"
+                  echo ""
+                  cat "$METRICS_FILE"
+              elif [[ "$path" == "/health" ]]; then
+                  # Health check endpoint
+                  echo "HTTP/1.1 200 OK"
+                  echo "Connection: close"
+                  echo "Content-Type: text/plain"
+                  echo "Content-Length: 3"
+                  echo ""
+                  echo "OK"
+              else
+                  # 404 for other paths
+                  echo "HTTP/1.1 404 Not Found"
+                  echo "Connection: close"
+                  echo "Content-Type: text/plain"
+                  echo "Content-Length: 10"
+                  echo ""
+                  echo "Not Found"
+              fi
+          }
+
+          handle_request
+        |||,
+      },
+
+      local configMap = k.core.v1.configMap,
+      configmap:
+        configMap.new(name, self.data),
+
+      local container = k.core.v1.container,
+      container::
+        container.new('static-exporter', image)
+        + container.withPorts([
+          k.core.v1.containerPort.newNamed(name='http-metrics', containerPort=port),
+        ])
+        + k.util.resourcesRequests('10m', '10Mi')
+        + container.withCommand([
+          'sh',
+          '-eu',
+          '-c',
+          |||
+            # handler is created in a new file
+            mkdir -p "%(bin_dir)s"
+            echo '#!'$(which sh) > "%(bin_dir)s/handler"
+            cat /data/handler >> "%(bin_dir)s/handler"
+            chmod +x %(bin_dir)s/handler
+
+            # run nc, which forks each handler in its own process
+            exec nc -p %(port)d -l -k -e "%(bin_dir)s/handler" 0.0.0.0
+          ||| % {
+            port: port,
+            bin_dir: '/home/nonroot/bin',
+          },
+        ]) +
+        container.securityContext.withRunAsUser(65532) +
+        container.securityContext.withRunAsGroup(65532) +
+        container.readinessProbe.httpGet.withPath('/health') +
+        container.readinessProbe.httpGet.withPort('http-metrics')
+      ,
+
+      local deployment = k.apps.v1.deployment,
+      local volumeMount = k.core.v1.volumeMount,
+      deployment:
+        deployment.new(name, replicas=1, containers=[self.container]),
+    },
+
+
   withData(data):: { data: data },
 
   withDataMixin(data):: { data+: data },