Make sure gauge for outbound queue is 0 on startup

[kristinapathak]

caduceus/metrics.go

Line 150 in 00ce0db

c.queueDepthGauge = m.NewGauge(OutgoingQueueDepth).With("url", c.id)

Instead of making new gauges and other metrics for each outbound sender, we should create these once for caduceus and then pass what is needed to the outbound senders.

Then, we can easily set the gauge to 0 on startup, which hopefully will fix the issue of the gauge getting stuck on a specific queue depth that doesn't look correct:

Edit: this is unlikely to fix the gauge issue, but would still be a nice fix to make.

[kristinapathak]

Setting the gauge to 0 for each label was done in this pr:
#205

Fixing the metrics to create counters and metrics once instead of in a variety of different structs can be done at a later time. This will probably be partially or completely done as a part of moving to uber.fx or creating a separate library for the outbound sender.