Add "(GPN: gcsfuse-<client-identifier>)" to useragent

[ashmeenkaur]

Description

Added GPN:gcsfuse- to user agent.
Removed redundant client identifier(appname or metadata image type env variable) from user agent if it has been appended to GPN.

Possible Scenarios and changes:

1. When both GCSFUSE_METADATA_IMAGE_TYPE environment variable and app-name flag are set
   • GCSFUSE_METADATA_IMAGE_TYPE=DLC app-name=myapp
   • Old UserAgent = gcsfuse/0.42.3 (Go version go1.19.5) myapp DLC,gzip(gfe)
   • New UserAgent =gcsfuse/0.42.3 (Go version go1.19.5) myapp (GPN:gcsfuse-DLC),gzip(gfe)
2. When both GCSFUSE_METADATA_IMAGE_TYPE environment variable and app-name flag are unset
   • GCSFUSE_METADATA_IMAGE_TYPE="" app-name=""
   • Old UserAgent = gcsfuse/0.42.3 (Go version go1.19.5),gzip(gfe)
   • New UserAgent =gcsfuse/0.42.3 (Go version go1.19.5) (GPN:gcsfuse),gzip(gfe)
3. When GCSFUSE_METADATA_IMAGE_TYPE is set and app-name is unset
   • GCSFUSE_METADATA_IMAGE_TYPE=DLC app-name=""
   • Old UserAgent = gcsfuse/0.42.3 (Go version go1.19.5) DLC,gzip(gfe)
   • New UserAgent =gcsfuse/0.42.3 (Go version go1.19.5) (GPN:gcsfuse-DLC),gzip(gfe)
4. When GCSFUSE_METADATA_IMAGE_TYPE is unset and app-name is set
   • GCSFUSE_METADATA_IMAGE_TYPE="" app-name=myapp
   • Old UserAgent = gcsfuse/0.42.3 (Go version go1.19.5) myapp,gzip(gfe)
   • New UserAgent =gcsfuse/0.42.3 (Go version go1.19.5) (GPN:gcsfuse-myapp),gzip(gfe)

Link to the issue in case of a bug fix.

Testing details

1. Manual - Manually verified that the user agent has GPN appended
2. Unit tests - added
3. Integration tests - NA

[sethiay]

Please add examples of final user agent string in each of the 4 combinations possible with app-name and env variable in the Description of this PR.