Metadata server address modification to account for default port

[srkethireddy]

Fixing the utilization of the GCE Metadata host server address environment variable to account for the case where the user does not specify a port (defaults to port 8080)

b/451639946

cc: @apolcyn, @anicr7

[ejona86]

/gcbrun

[ejona86]

@apolcyn, @anicr7, the logic here has become more subtle. We're supposed to release tomorrow, and this still seems a few days away from full testing. I'm wary of including it is the release.

[srkethireddy]

Changed the utilization of the GCE Metadata host server address environment variable so that if a port is specified or unspecified, it defaults to use port 8080 for ALTS handshake.

Tested scenarios where environment variable was not set, environment variable was set but with no port, and environment variable was set with a port other than 8080

[ejona86]

/gcbrun

[ejona86]

The testing got completed sooner than originally believed, so I'm good with this now.