grpc-js: fix target in tracing logs

[badsyntax]

I'm attempting to debug a random "failed to parse server response" error and this is what i'm seeing in my logs (note the [object Object]):

2020-04-23T06:13:06.747Z | dns_resolver | Resolved addresses for target [object Object]: [::1:57426,127.0.0.1:57426]
2020-04-23T06:13:06.747Z | pick_first | IDLE -> IDLE
2020-04-23T06:13:06.748Z | resolving_load_balancer | [object Object] CONNECTING -> IDLE
2020-04-23T06:13:06.748Z | connectivity_state | [object Object] CONNECTING -> IDLE
2020-04-23T06:13:06.748Z | dns_resolver | Resolution update requested for target [object Object]
2020-04-23T06:13:06.748Z | resolving_load_balancer | [object Object] IDLE -> CONNECTING
2020-04-23T06:13:06.748Z | connectivity_state | [object Object] IDLE -> CONNECTING
2020-04-23T06:13:06.748Z | resolving_load_balancer | [object Object] CONNECTING -> CONNECTING

I guess this is related to #1364

[murgatroid99]

I noticed some of this yesterday, but I didn't get around to dealing with it. Thank you for fixing it.