feat(spanner): support user-provided OpenTelemetry for client metrics export #22900

Job	Run time
bulk-filter	1m 10s
gapic-libraries-bom	18m 49s
windows	2m 4s
lint	3m 19s
units (8)	48s
units (26)	47s
units (25)	34s
enforcer	20m 57s
units (21)	35s
units (11)	38s
units (17)	32s
changes	28s
split-clirr (java-spanner)	2m 13s
split-units (java-spanner, 8)	5m 18s
split-units (java-spanner, 26)	4m 16s
split-units (java-spanner, 21)	4m 31s
split-units (java-spanner, 11)	5m 6s
split-dependencies (java-spanner)	3m 11s
split-units (java-spanner, 25)	6h 0m 17s
split-units (java-spanner, 17)	4m 32s
conditional-required-check	4s
	7h 20m 9s

Provide feedback