Legacy decode key fails initially for protobufjs 7.5

[matt-blanchette]

Steps:

1. Upgrade from 9.2.1 to 10.0.1, using latest google-gax 5.0.3 and latest protobufjs 7.5.3
2. Datastore.keyFromLegacyUrlsafe fails on first call with Error: invalid wire type 4 at offset 43
3. Subsequent calls to keyFromLegacyUrlsafe are successful after the first failure

Cause:

Protobufjs version 7.5.0 and later throws the invalid wire type error.

Workaround:

Downgrade to earlier Protobufjs version 7.4.0 or 7.3.3