go/vt/mysqlctl/backup.go: add backup/restore duration stats

[guidoiaquinti]

Description

Add metrics to track backup/restore operations duration.

Related Issue(s)

N/A

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

Test

Before

vitess@47039963e297:/vt/src/vitess.io/vitess/examples/local$ curl -i -s localhost:15101/metrics | grep vttablet_backup
# HELP vttablet_backup_duration_seconds How long it took to complete the last backup operation (in seconds)
# TYPE vttablet_backup_duration_seconds gauge
vttablet_backup_duration_seconds 0

vitess@47039963e297:/vt/src/vitess.io/vitess/examples/local$ vtctlclient Backup zone1-0000000101
I0218 12:55:41.952392   48318 main.go:67] I0218 12:55:41.951644 backup.go:163] I0218 12:55:41.947789 builtinbackupengine.go:141] Hook: , Compress: true
I0218 12:55:41.952878   48318 main.go:67] I0218 12:55:41.951707 backup.go:163] I0218 12:55:41.950990 builtinbackupengine.go:151] getting current replication status
I0218 12:55:41.961514   48318 main.go:67] I0218 12:55:41.960952 backup.go:163] I0218 12:55:41.960720 builtinbackupengine.go:192] using replication position: 41d2b95c-71e8-11eb-a990-0242ac110002:1-20
[...]

After

# HELP vttablet_backup_duration_seconds How long it took to complete the last backup operation (in seconds)
# TYPE vttablet_backup_duration_seconds gauge
vttablet_backup_duration_seconds 5

[deepthi]

LGTM