fix(bigtable): thread visibility on heartbeatInterval #20652

Job	Run time
windows	2m 37s
lint	3m 42s
enforcer	20m 29s
generation-config-check	1m 38s
gapic-libraries-bom	18m 34s
changes	1m 27s
bulk-filter	1m 12s
units (25)	39s
units (17)	30s
units (21)	28s
units (11)	32s
units (8)	49s
split-clirr (java-bigtable)	2m 30s
split-dependencies (java-bigtable)	5m 17s
split-units (java-bigtable, 8)	5m 39s
split-units (java-bigtable, 21)	6m 45s
split-units (java-bigtable, 11)	6m 20s
split-units (java-bigtable, 17)	7m 28s
split-units (java-bigtable, 25)	7m 17s
conditional-required-check	3s
	1h 33m 56s

Provide feedback