Fix internal metrics for output split into multiple lines

[danielnelson]

Fix a issue where the internal metrics were split over two lines unnecessarily due to the alias tag being set to the empty string.

1.13.4

> internal_write,host=loaner,output=kafka,version=unknown errors=0i,metrics_filtered=0i,write_time_ns=0i 1583447470000000000
> internal_write,host=loaner,output=kafka,version=unknown buffer_limit=10000000i,buffer_size=0i,metrics_added=0i,metrics_dropped=0i,metrics_written=0i 1583447470000000000

With change

internal_write,host=loaner,output=file,version=unknown buffer_limit=10000000i,buffer_size=0i,errors=0i,metrics_added=0i,metrics_dropped=0i,metrics_filtered=0i,metrics_written=0i,write_time_ns=0i 1583447704000000000

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[ssoroka]

Nice catch. We might want to add a test to avoid a regression