The plugin currently only provides data from the "Unit" dbus interface. On my system that's

AccessSELinuxContext ConditionTimestampMonotonic JobTimeoutUSec Requisite
ActivationDetails Conditions JoinsNamespaceOf RequisiteOf
ActiveEnterTimestamp ConflictedBy LoadError SliceOf
ActiveEnterTimestampMonotonic Conflicts LoadState SourcePath
ActiveExitTimestamp ConsistsOf Markers StartLimitAction
ActiveExitTimestampMonotonic DefaultDependencies Names StartLimitBurst
ActiveState Description NeedDaemonReload StartLimitIntervalUSec
After Documentation OnFailure StateChangeTimestamp
AllowIsolate DropInPaths OnFailureJobMode StateChangeTimestampMonotonic
AssertResult FailureAction OnFailureOf StopPropagatedFrom
AssertTimestamp FailureActionExitStatus OnSuccess StopWhenUnneeded
AssertTimestampMonotonic Following OnSuccessJobMode SubState
Asserts FragmentPath OnSuccessOf SuccessAction
Before FreezerState PartOf SuccessActionExitStatus
BindsTo Id Perpetual Transient
BoundBy IgnoreOnIsolate PropagatesReloadTo TriggeredBy
CanClean InactiveEnterTimestamp PropagatesStopTo Triggers
CanFreeze InactiveEnterTimestampMonotonic RebootArgument UnitFilePreset
CanIsolate InactiveExitTimestamp Refs UnitFileState
CanReload InactiveExitTimestampMonotonic RefuseManualStart UpheldBy
CanStart InvocationID RefuseManualStop Upholds
CanStop Job ReloadPropagatedFrom WantedBy
CollectMode JobRunningTimeoutUSec RequiredBy Wants
ConditionResult JobTimeoutAction Requires
ConditionTimestamp JobTimeoutRebootArgument RequiresMountsFor

I would be interesting in some of the other bits systemctl show has to offer for me, that come from the interface "Service":

AllowedCPUs Group ManagedOOMMemoryPressureLimit RuntimeDirectoryPreserve
AllowedMemoryNodes GuessMainPID ManagedOOMPreference RuntimeDirectorySymlink
AmbientCapabilities IOAccounting ManagedOOMSwap RuntimeMaxUSec
AppArmorProfile IODeviceLatencyTargetUSec MemoryAccounting RuntimeRandomizedExtraUSec
BPFProgram IODeviceWeight MemoryAvailable SELinuxContext
BindPaths IOReadBandwidthMax MemoryCurrent SameProcessGroup
BindReadOnlyPaths IOReadBytes MemoryDenyWriteExecute SecureBits
BlockIOAccounting IOReadIOPSMax MemoryHigh SendSIGHUP
BlockIODeviceWeight IOReadOperations MemoryLimit SendSIGKILL
BlockIOReadBandwidth IOSchedulingClass MemoryLow SetCredential
BlockIOWeight IOSchedulingPriority MemoryMax SetCredentialEncrypted
BlockIOWriteBandwidth IOWeight MemoryMin Slice
BusName IOWriteBandwidthMax MemorySwapMax SmackProcessLabel
CPUAccounting IOWriteBytes MountAPIVFS SocketBindAllow
CPUAffinity IOWriteIOPSMax MountFlags SocketBindDeny
CPUAffinityFromNUMA IOWriteOperations MountImages StandardError
CPUQuotaPerSecUSec IPAccounting NFileDescriptorStore StandardErrorFileDescriptorName
CPUQuotaPeriodUSec IPAddressAllow NRestarts StandardInput
CPUSchedulingPolicy IPAddressDeny NUMAMask StandardInputData
CPUSchedulingPriority IPCNamespacePath NUMAPolicy StandardInputFileDescriptorName
CPUSchedulingResetOnFork IPEgressBytes NetworkNamespacePath StandardOutput
CPUShares IPEgressFilterPath Nice StandardOutputFileDescriptorName
CPUUsageNSec IPEgressPackets NoExecPaths StartupAllowedCPUs
CPUWeight IPIngressBytes NoNewPrivileges StartupAllowedMemoryNodes
CacheDirectory IPIngressFilterPath NonBlocking StartupBlockIOWeight
CacheDirectoryMode IPIngressPackets NotifyAccess StartupCPUShares
CacheDirectorySymlink IgnoreSIGPIPE OOMPolicy StartupCPUWeight
CapabilityBoundingSet InaccessiblePaths OOMScoreAdjust StartupIOWeight
CleanResult KeyringMode PAMName StateDirectory
ConfigurationDirectory KillMode PIDFile StateDirectoryMode
ConfigurationDirectoryMode KillSignal PassEnvironment StateDirectorySymlink
ControlGroup LimitAS Personality StatusErrno
ControlGroupId LimitASSoft PrivateDevices StatusText
ControlPID LimitCORE PrivateIPC SuccessExitStatus
CoredumpFilter LimitCORESoft PrivateMounts SupplementaryGroups
DefaultMemoryLow LimitCPU PrivateNetwork SyslogFacility
DefaultMemoryMin LimitCPUSoft PrivateTmp SyslogIdentifier
Delegate LimitDATA PrivateUsers SyslogLevel
DelegateControllers LimitDATASoft ProcSubset SyslogLevelPrefix
DeviceAllow LimitFSIZE ProtectClock SyslogPriority
DevicePolicy LimitFSIZESoft ProtectControlGroups SystemCallArchitectures
DisableControllers LimitLOCKS ProtectHome SystemCallErrorNumber
DynamicUser LimitLOCKSSoft ProtectHostname SystemCallFilter
EffectiveCPUs LimitMEMLOCK ProtectKernelLogs SystemCallLog
EffectiveMemoryNodes LimitMEMLOCKSoft ProtectKernelModules TTYColumns
Environment LimitMSGQUEUE ProtectKernelTunables TTYPath
EnvironmentFiles LimitMSGQUEUESoft ProtectProc TTYReset
ExecCondition LimitNICE ProtectSystem TTYRows
ExecConditionEx LimitNICESoft ReadOnlyPaths TTYVHangup
ExecMainCode LimitNOFILE ReadWritePaths TTYVTDisallocate
ExecMainExitTimestamp LimitNOFILESoft ReloadResult TasksAccounting
ExecMainExitTimestampMonotonic LimitNPROC RemainAfterExit TasksCurrent
ExecMainPID LimitNPROCSoft RemoveIPC TasksMax
ExecMainStartTimestamp LimitRSS Restart TemporaryFileSystem
ExecMainStartTimestampMonotonic LimitRSSSoft RestartForceExitStatus TimeoutAbortUSec
ExecMainStatus LimitRTPRIO RestartKillSignal TimeoutCleanUSec
ExecPaths LimitRTPRIOSoft RestartPreventExitStatus TimeoutStartFailureMode
ExecReload LimitRTTIME RestartUSec TimeoutStartUSec
ExecReloadEx LimitRTTIMESoft RestrictAddressFamilies TimeoutStopFailureMode
ExecSearchPath LimitSIGPENDING RestrictFileSystems TimeoutStopUSec
ExecStart LimitSIGPENDINGSoft RestrictNamespaces TimerSlackNSec
ExecStartEx LimitSTACK RestrictNetworkInterfaces Type
ExecStartPost LimitSTACKSoft RestrictRealtime UID
ExecStartPostEx LoadCredential RestrictSUIDSGID UMask
ExecStartPre LoadCredentialEncrypted Result USBFunctionDescriptors
ExecStartPreEx LockPersonality RootDirectory USBFunctionStrings
ExecStop LogExtraFields RootDirectoryStartOnly UnsetEnvironment
ExecStopEx LogLevelMax RootHash User
ExecStopPost LogNamespace RootHashPath UtmpIdentifier
ExecStopPostEx LogRateLimitBurst RootHashSignature UtmpMode
ExitType LogRateLimitIntervalUSec RootHashSignaturePath WatchdogSignal
ExtensionDirectories LogsDirectory RootImage WatchdogTimestamp
ExtensionImages LogsDirectoryMode RootImageOptions WatchdogTimestampMonotonic
FileDescriptorStoreMax LogsDirectorySymlink RootVerity WatchdogUSec
FinalKillSignal MainPID RuntimeDirectory WorkingDirectory
GID ManagedOOMMemoryPressure RuntimeDirectoryMode

I'm particularly interested in MemoryCurrent at the moment. I know that I can gather this information from cgroupfs too, knowing the name of the unit, but it feels clumsy to do so. The actual items exposed in this interface might depend on the cgroup accounting settings on the system. Permissions don't seem to be much of a blocker, looking at the ssh service as the user zabbix here:

zabbix@myserver:~$ busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1/unit/ssh_2eservice org.freedesktop.systemd1.Service MemoryCurrent
t 7520256

I propose an item like the existing unit.info that prototype items can depend on.