kg: add new handler for rendering the topology graph

docker: add missing fonts for rasterized graphviz  output formats

add missing license header

kg: do not export handlers

use http package for status codes

keep checks for errors in a single line

simplify error message about failed invocation of dot

pass node hostname and subnet to graph handler

use SVG as default format for graph handler

register health handler with HandleFunc

add option for selecting layout to graph handler and using circo as new default

e2e: add tests for HTTP handlers

e2e: fix and simplify handler tests

add should comments to assertions

e2s: use assert_fail instead of assert _not

add missing mime-type header for graph handler

use switch/case statements for validating formats / layouts

e2e: fix handlers tests

Co-authored-by: leonnicolas <60091705+leonnicolas@users.noreply.github.com>

graph-handler: add missing font to Dockerfile

Dockerfile: remove unnecessary font

This commit leaves Noto as the only font package, as one font package is
sufficient for the container.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

e2e/handlers.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# shellcheck disable=SC1091
+. lib.sh
+
+setup_suite() {
+	# shellcheck disable=SC2016
+	block_until_ready_by_name kube-system kilo-userspace 
+	_kubectl wait pod -l app.kubernetes.io/name=adjacency --for=condition=Ready --timeout 3m
+}
+
+test_graph_handler() {
+    assert "curl_pod 'http://10.4.0.1:1107/graph?format=svg&layout=circo' | grep -q '<svg'" "graph handler should produce SVG output"
+    assert "curl_pod http://10.4.0.1:1107/graph?layout=circo | grep -q '<svg'" "graph handler should default to SVG output"
+    assert "curl_pod http://10.4.0.1:1107/graph | grep -q '<svg'" "graph handler should default to SVG output"
+    assert_fail "curl_pod http://10.4.0.1:1107/graph?layout=fake | grep -q '<svg'" "graph handler should reject invalid layout"
+    assert_fail "curl_pod http://10.4.0.1:1107/graph?format=fake | grep -q '<svg'" "graph handler should reject invalid format"
+}
+
+test_health_handler() {
+    assert "curl_pod http://10.4.0.1:1107/health" "health handler should return a status code of 200"
+}
+
+test_metrics_handler() {
+    assert "curl_pod http://10.4.0.1:1107/metrics" "metrics handler should return a status code of 200"
+    assert "(( $(curl_pod http://10.4.0.1:1107/metrics | grep -E ^kilo_nodes | cut -d " " -f 2) > 0 ))" "metrics handler should provide metric: kilo_nodes > 0"
+}