TUN-8731: Implement diag/system endpoint

## Summary
This PR will add a new endpoint, "diag/system" to the metrics server that collects system information from different operating systems.

Closes TUN-8731

diagnostic/handlers_test.go

@@ -0,0 +1,108 @@
+package diagnostic_test
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/rs/zerolog"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/cloudflare/cloudflared/diagnostic"
+)
+
+type SystemCollectorMock struct{}
+
+const (
+	systemInformationKey = "sikey"
+	rawInformationKey    = "rikey"
+	errorKey             = "errkey"
+)
+
+func setCtxValuesForSystemCollector(
+	systemInfo *diagnostic.SystemInformation,
+	rawInfo string,
+	err error,
+) context.Context {
+	ctx := context.Background()
+	ctx = context.WithValue(ctx, systemInformationKey, systemInfo)
+	ctx = context.WithValue(ctx, rawInformationKey, rawInfo)
+	ctx = context.WithValue(ctx, errorKey, err)
+
+	return ctx
+}
+
+func (*SystemCollectorMock) Collect(ctx context.Context) (*diagnostic.SystemInformation, string, error) {
+	si, _ := ctx.Value(systemInformationKey).(*diagnostic.SystemInformation)
+	ri, _ := ctx.Value(rawInformationKey).(string)
+	err, _ := ctx.Value(errorKey).(error)
+
+	return si, ri, err
+}
+
+func TestSystemHandler(t *testing.T) {
+	t.Parallel()
+
+	log := zerolog.Nop()
+	tests := []struct {
+		name       string
+		systemInfo *diagnostic.SystemInformation
+		rawInfo    string
+		err        error
+		statusCode int
+	}{
+		{
+			name: "happy path",
+			systemInfo: diagnostic.NewSystemInformation(
+				0, 0, 0, 0,
+				"string", "string", "string", "string",
+				"string", "string", nil,
+			),
+			rawInfo:    "",
+			err:        nil,
+			statusCode: http.StatusOK,
+		},
+		{
+			name: "on error and raw info", systemInfo: nil,
+			rawInfo: "raw info", err: errors.New("an error"), statusCode: http.StatusOK,
+		},
+		{
+			name: "on error and no raw info", systemInfo: nil,
+			rawInfo: "", err: errors.New("an error"), statusCode: http.StatusInternalServerError,
+		},
+		{
+			name: "malformed response", systemInfo: nil, rawInfo: "", err: nil, statusCode: http.StatusInternalServerError,
+		},
+	}
+
+	for _, tCase := range tests {
+		t.Run(tCase.name, func(t *testing.T) {
+			t.Parallel()
+			handler := diagnostic.NewDiagnosticHandler(&log, 0, &SystemCollectorMock{})
+			recorder := httptest.NewRecorder()
+			ctx := setCtxValuesForSystemCollector(tCase.systemInfo, tCase.rawInfo, tCase.err)
+			request, err := http.NewRequestWithContext(ctx, http.MethodGet, "/diag/syste,", nil)
+			require.NoError(t, err)
+			handler.SystemHandler(recorder, request)
+
+			assert.Equal(t, tCase.statusCode, recorder.Code)
+			if tCase.statusCode == http.StatusOK && tCase.systemInfo != nil {
+				var response diagnostic.SystemInformation
+
+				decoder := json.NewDecoder(recorder.Body)
+				err = decoder.Decode(&response)
+				require.NoError(t, err)
+				assert.Equal(t, tCase.systemInfo, &response)
+			} else if tCase.statusCode == http.StatusOK && tCase.rawInfo != "" {
+				rawBytes, err := io.ReadAll(recorder.Body)
+				require.NoError(t, err)
+				assert.Equal(t, tCase.rawInfo, string(rawBytes))
+			}
+		})
+	}
+}