Istio-proxy router and http log meanings

I am trying to debug why some requests are failing and increased the logging level with the command below:

istioctl proxy-config log kubectl select pod --level http:debug,router:debug

And below are the logs:

2020-07-04T13:44:47.217743Z	debug	envoy http	[external/envoy/source/common/http/] [C10659][S779203182681781930] request headers complete (end_stream=true):
':authority', ''
':path', '/items'
':method', 'GET'
'user-agent', 'curl/7.64.1'
'accept', '*/*'

2020-07-04T13:44:47.217791Z	debug	envoy http	[external/envoy/source/common/http/] [C10659][S779203182681781930] request end stream
2020-07-04T13:44:47.217915Z	debug	envoy router	[external/envoy/source/common/router/] [C10659][S779203182681781930] cluster 'outbound|80|version-v2|catalog.istioinaction.svc.cluster.local' match for URL '/items'
2020-07-04T13:44:47.217983Z	debug	envoy http	[external/envoy/source/common/http/] [C10659][S779203182681781930] Sending local reply with details no_healthy_upstream
2020-07-04T13:44:47.218040Z	debug	envoy http	[external/envoy/source/common/http/] [C10659][S779203182681781930] encoding headers via codec (end_stream=false):
':status', '503'
'content-length', '19'
'content-type', 'text/plain'
'date', 'Sat, 04 Jul 2020 13:44:47 GMT'
'server', 'istio-envoy'

Sections which I was not able to find documentation on either on or in is the meaning of this section [C10659][S779203182681781930]

Is this a request identifier of some sorts? (As it would make sense), can you link me to some docs explaining this?

And last question that really irks me is how (if possible) would I correlate this with access_logs?