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/system_collector_linux.go

@@ -0,0 +1,120 @@
+//go:build linux
+
+package diagnostic
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+	"strconv"
+	"strings"
+)
+
+type SystemCollectorImpl struct {
+	version string
+}
+
+func NewSystemCollectorImpl(
+	version string,
+) *SystemCollectorImpl {
+	return &SystemCollectorImpl{
+		version,
+	}
+}
+
+func (collector *SystemCollectorImpl) Collect(ctx context.Context) (*SystemInformation, string, error) {
+	memoryInfo, memoryInfoRaw, memoryInfoErr := collectMemoryInformation(ctx)
+	fdInfo, fdInfoRaw, fdInfoErr := collectFileDescriptorInformation(ctx)
+	disks, disksRaw, diskErr := collectDiskVolumeInformationUnix(ctx)
+	osInfo, osInfoRaw, osInfoErr := collectOSInformationUnix(ctx)
+
+	if memoryInfoErr != nil {
+		raw := RawSystemInformation(osInfoRaw, memoryInfoRaw, fdInfoRaw, disksRaw)
+		return nil, raw, memoryInfoErr
+	}
+
+	if fdInfoErr != nil {
+		raw := RawSystemInformation(osInfoRaw, memoryInfoRaw, fdInfoRaw, disksRaw)
+		return nil, raw, fdInfoErr
+	}
+
+	if diskErr != nil {
+		raw := RawSystemInformation(osInfoRaw, memoryInfoRaw, fdInfoRaw, disksRaw)
+		return nil, raw, diskErr
+	}
+
+	if osInfoErr != nil {
+		raw := RawSystemInformation(osInfoRaw, memoryInfoRaw, fdInfoRaw, disksRaw)
+		return nil, raw, osInfoErr
+	}
+
+	return NewSystemInformation(
+		memoryInfo.MemoryMaximum,
+		memoryInfo.MemoryCurrent,
+		fdInfo.FileDescriptorMaximum,
+		fdInfo.FileDescriptorCurrent,
+		osInfo.OsSystem,
+		osInfo.Name,
+		osInfo.OsVersion,
+		osInfo.OsRelease,
+		osInfo.Architecture,
+		collector.version,
+		disks,
+	), "", nil
+}
+
+func collectMemoryInformation(ctx context.Context) (*MemoryInformation, string, error) {
+	// This function relies on the output of `cat /proc/meminfo` to retrieve
+	// memoryMax and  memoryCurrent.
+	// The expected output is in the format of `KEY VALUE UNIT`.
+	const (
+		memTotalPrefix     = "MemTotal"
+		memAvailablePrefix = "MemAvailable"
+	)
+
+	command := exec.CommandContext(ctx, "cat", "/proc/meminfo")
+
+	stdout, err := command.Output()
+	if err != nil {
+		return nil, "", fmt.Errorf("error retrieving output from command '%s': %w", command.String(), err)
+	}
+
+	output := string(stdout)
+
+	mapper := func(field string) (uint64, error) {
+		field = strings.TrimRight(field, " kB")
+
+		return strconv.ParseUint(field, 10, 64)
+	}
+
+	memoryInfo, err := ParseMemoryInformationFromKV(output, memTotalPrefix, memAvailablePrefix, mapper)
+	if err != nil {
+		return nil, output, err
+	}
+
+	// returning raw output in case other collected information
+	// resulted in errors
+	return memoryInfo, output, nil
+}
+
+func collectFileDescriptorInformation(ctx context.Context) (*FileDescriptorInformation, string, error) {
+	// Command retrieved from https://docs.kernel.org/admin-guide/sysctl/fs.html#file-max-file-nr.
+	// If the sysctl is not available the command with fail.
+	command := exec.CommandContext(ctx, "sysctl", "-n", "fs.file-nr")
+
+	stdout, err := command.Output()
+	if err != nil {
+		return nil, "", fmt.Errorf("error retrieving output from command '%s': %w", command.String(), err)
+	}
+
+	output := string(stdout)
+
+	fileDescriptorInfo, err := ParseSysctlFileDescriptorInformation(output)
+	if err != nil {
+		return nil, output, err
+	}
+
+	// returning raw output in case other collected information
+	// resulted in errors
+	return fileDescriptorInfo, output, nil
+}