fix: fix dropReason metrics labels

[spikecurtis]

the counter_packets_dropped_reason metric is using incorrect labels, probably because a reason was dropped and a new one added to the end, causing the reasons between to be shifted out of alignment with their labels in the metrics.

The actual reasons are

const (
	dropReasonUnknownDest      dropReason = iota // unknown destination pubkey
	dropReasonUnknownDestOnFwd                   // unknown destination pubkey on a derp-forwarded packet
	dropReasonGoneDisconnected                   // destination tailscaled disconnected before we could send
	dropReasonQueueHead                          // destination queue is full, dropped packet at queue head
	dropReasonQueueTail                          // destination queue is full, dropped packet at queue tail
	dropReasonWriteError                         // OS write() failed
	dropReasonDupClient                          // the public key is connected 2+ times (active/active, fighting)
)

I noticed this while looking into DERP performance, since I was seeing a bunch of drops for gone_not_here which is not a valid reason in present version.

[andrew-d]

@spikecurtis hi there, Andrew from Tailscale here 👋 Any chance I could ask you to submit this PR upstream? Or give me permission to pick this commit into a PR, maintaining authorship information? Good find 😄