Test PVC Name and Namespace included in the volume metrics

Test step

Given created 2 volumes (volume-1, volume-2)

When PVC created for volume (volume-1) And attached volumes (volume-1, volume-2)

Then metrics with longhorn_volume_ prefix should include pvc="volume-1"

curl -sSL http://10.0.2.212:32744/metrics | grep longhorn_volume | grep ip-10-0-2-151 | grep volume-1
longhorn_volume_actual_size_bytes{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 0
longhorn_volume_capacity_bytes{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 1.073741824e+09
longhorn_volume_read_iops{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 0
longhorn_volume_read_latency{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 0
longhorn_volume_read_throughput{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 0
longhorn_volume_robustness{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 1
longhorn_volume_state{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 2
longhorn_volume_write_iops{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 0
longhorn_volume_write_latency{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 0
longhorn_volume_write_throughput{pvc_namespace="default",node="ip-10-0-2-151",pvc="volume-1",volume="volume-1"} 0

And metrics with longhorn_volume_ prefix should include pvc="" for (volume-2)

> curl -sSL http://10.0.2.212:32744/metrics | grep longhorn_volume | grep ip-10-0-2-151 | grep volume-2
longhorn_volume_actual_size_bytes{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 0
longhorn_volume_capacity_bytes{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 1.073741824e+09
longhorn_volume_read_iops{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 0
longhorn_volume_read_latency{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 0
longhorn_volume_read_throughput{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 0
longhorn_volume_robustness{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 1
longhorn_volume_state{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 2
longhorn_volume_write_iops{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 0
longhorn_volume_write_latency{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 0
longhorn_volume_write_throughput{pvc_namespace="",node="ip-10-0-2-151",pvc="",volume="volume-2"} 0
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