[pkg/ottl] Unexport the grammar's constants (#29925) #22

Job	Run time
windows-unittest-matrix (receiver-0)	14m 57s
windows-unittest-matrix (receiver-1)	14m 44s
windows-unittest-matrix (processor)	13m 41s
windows-unittest-matrix (exporter)	15m 42s
windows-unittest-matrix (extension)	12m 49s
windows-unittest-matrix (internal)	12m 28s
windows-unittest-matrix (pkg)	12m 5s
windows-unittest-matrix (cmd)	15m 42s
windows-unittest-matrix (other)	15m 40s
windows-unittest	0s
	2h 7m 48s

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