[NC | NSFS] Fix NSFS prometheus write bytes

[tangledbytes]

Explain the changes

This PR fixes missing write bytes from the prometheus metrics

Example

# HELP NooBaa_Endpoint_hub_read_bytes hub read bytes in namespace cache bucket
# TYPE NooBaa_Endpoint_hub_read_bytes counter
NooBaa_Endpoint_hub_read_bytes{bucket_name="first.bucket"} 899606

# HELP NooBaa_Endpoint_hub_write_bytes hub write bytes in namespace cache bucket
# TYPE NooBaa_Endpoint_hub_write_bytes counter
NooBaa_Endpoint_hub_write_bytes{bucket_name="first.bucket"} 1349409

NOTE: This PR was tested with #7562 as well.

Issues: Fixed #xxx / Gap #xxx

Previously, prometheus metrics lacked write bytes related info.

Testing Instructions:

1. Start NooBaa (NSFS) with metrics server
2. Open the webpage and find the write bytes as mentioned in the above example.

• Doc added/updated
• Tests added

[naveenpaul1]

LGTM