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.go

@@ -0,0 +1,83 @@
+package diagnostic
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"time"
+
+	"github.com/rs/zerolog"
+)
+
+type Handler struct {
+	log             *zerolog.Logger
+	timeout         time.Duration
+	systemCollector SystemCollector
+}
+
+func NewDiagnosticHandler(
+	log *zerolog.Logger,
+	timeout time.Duration,
+	systemCollector SystemCollector,
+) *Handler {
+	if timeout == 0 {
+		timeout = defaultCollectorTimeout
+	}
+
+	return &Handler{
+		log,
+		timeout,
+		systemCollector,
+	}
+}
+
+func (handler *Handler) SystemHandler(writer http.ResponseWriter, request *http.Request) {
+	logger := handler.log.With().Str(collectorField, systemCollectorName).Logger()
+	logger.Info().Msg("Collection started")
+
+	defer func() {
+		logger.Info().Msg("Collection finished")
+	}()
+
+	ctx, cancel := context.WithTimeout(request.Context(), handler.timeout)
+
+	defer cancel()
+
+	info, rawInfo, err := handler.systemCollector.Collect(ctx)
+	if err != nil {
+		logger.Error().Err(err).Msg("error occurred whilst collecting system information")
+
+		if rawInfo != "" {
+			logger.Info().Msg("using raw information fallback")
+			bytes := []byte(rawInfo)
+			writeResponse(writer, bytes, &logger)
+		} else {
+			logger.Error().Msg("no raw information available")
+			writer.WriteHeader(http.StatusInternalServerError)
+		}
+
+		return
+	}
+
+	if info == nil {
+		logger.Error().Msgf("system information collection is nil")
+		writer.WriteHeader(http.StatusInternalServerError)
+	}
+
+	encoder := json.NewEncoder(writer)
+
+	err = encoder.Encode(info)
+	if err != nil {
+		logger.Error().Err(err).Msgf("error occurred whilst serializing information")
+		writer.WriteHeader(http.StatusInternalServerError)
+	}
+}
+
+func writeResponse(writer http.ResponseWriter, bytes []byte, logger *zerolog.Logger) {
+	bytesWritten, err := writer.Write(bytes)
+	if err != nil {
+		logger.Error().Err(err).Msg("error occurred writing response")
+	} else if bytesWritten != len(bytes) {
+		logger.Error().Msgf("error incomplete write response %d/%d", bytesWritten, len(bytes))
+	}
+}