[receiver/splunk_hec] Fix single metric value parsing (#33085) #12

Job	Run time
windows-unittest-matrix (receiver-0)	19m 38s
windows-unittest-matrix (receiver-1)	24m 11s
windows-unittest-matrix (receiver-2)	24m 19s
windows-unittest-matrix (receiver-3)	21m 51s
windows-unittest-matrix (processor-0)	17m 26s
windows-unittest-matrix (processor-1)	16m 20s
windows-unittest-matrix (exporter-0)	14m 0s
windows-unittest-matrix (exporter-1)	26m 16s
windows-unittest-matrix (exporter-2)	19m 17s
windows-unittest-matrix (exporter-3)	21m 38s
windows-unittest-matrix (extension)	22m 5s
windows-unittest-matrix (connector)	15m 23s
windows-unittest-matrix (internal)	24m 1s
windows-unittest-matrix (pkg)	10m 30s
windows-unittest-matrix (cmd-0)	24m 11s
windows-unittest-matrix (cmd-1)	17m 59s
windows-unittest-matrix (other)	28m 51s
windows-unittest	3s
	5h 47m 59s

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