cmd/scollector: fix empty host= tags not working after PR #1856

[gbrayut]

The add function in cmd/scollector/collectors/collectors.go uses host= to indicate that tag should be omitted

See

bosun/cmd/scollector/collectors/collectors.go

Line 292 in ac7f667

// automatically added. If the value of the host key is the empty string, it

This should fix the vsphere errors on ny-bosun01:

error: collectors.go:256: Invalid tagset discovered: {disk=CO-EQL,host=}. Skipping datapoint. Added from: vsphere.go:138
error: collectors.go:256: Invalid tagset discovered: {disk=datastore1,host=}. Skipping datapoint. Added from: vsphere.go:105
error: collectors.go:256: Invalid tagset discovered: {disk=datastore1,host=}. Skipping datapoint. Added from: vsphere.go:136
error: collectors.go:256: Invalid tagset discovered: {disk=datastore1,host=}. Skipping datapoint. Added from: vsphere.go:138

[captncraig]

My only worry is that the invariant for Clean is now violated. Clean should always result in a valid datapoint (can be sent and accepted by opentsdb as is), or an error. A solution more specific to AddTS might be to move the host fixing up above the call to Clean.

[gbrayut]

Ah... I couldn't find that (bit scatter brained). Let me try that instead.

[gbrayut]

Yep... that works and is much better. I'll let this run on ny-bosun01 overnight as well but don't expect any issues.

Actually I'm going to merge this now since it is a trivial change of order of operations and it make testing the other branch easier.