spanner: encodeValue should prefer the Encoder interface before using generic proto serialization #10662
Description
Client
spanner
Expected behavior
When encodeValue encounters a proto message that implements the Encoder interface, EncodeSpanner should be used before generic proto encoding.
Actual behavior
encodeValue assumes that proto message types are being stored in a proto field type. This breaks backwards compatibility for custom types we've been using.
Additional context
e.g. Started after upgrading to v1.62.0