[chore][metricbeat/test_diskio]: fix CI failure

[VihasMakwana]

For diskio metricset,

beats/metricbeat/module/system/diskio/diskio.go

Lines 118 to 120 in bfaa70f

	if counters.SerialNumber != "" {
	event["serial_number"] = counters.SerialNumber
	}

serial_number field is set, conditionally.

This error has likely originated by bumping PR on https://github.com/elastic/elastic-agent-system-metrics.

I think, for such conditional fields, our best case is to remove them from event or else they trigger a failure.

We cannot add such fields at

beats/metricbeat/module/system/test_system.py

Lines 70 to 76 in bfaa70f

	SYSTEM_DISKIO = {
	metricbeat.P_DEF: ["name", "read.count", "write.count", "read.bytes",
	"write.bytes", "read.time", "write.time"]
	}
	SYSTEM_DISKIO[metricbeat.P_LINUX] = SYSTEM_DISKIO[metricbeat.P_DEF] + \
	["io.time", "io.ops"]

as that would also result in a failure, because the serial_number can be absent for some devices.

This is needed to unblock #41524

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @VihasMakwana? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.