[trace] Add logging support to opentracing plugins

[ajm188]

Description

This PR adds a new flag to package trace to initialize the opentracing tracer (either jaeger or datadog) with a logger attached. I've also introduced a struct that implements both jaeger's and datadog's logger interfaces using the go/vt/log package under the hood.

Demo

jaeger + JAEGER_REPORTER_LOG_SPANS=true + -tracing-enable-logging

I0608 12:08:21.671954       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:aaaaaaaaaaaaaaaa:bbbbbbbbbbbbbbbb:1
I0608 12:08:21.671978       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:bbbbbbbbbbbbbbbb:cccccccccccccccc:1
I0608 12:08:21.672031       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:dddddddddddddddd:eeeeeeeeeeeeeeee:1
I0608 12:08:21.765381       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:ffffffffffffffff:gggggggggggggggg:1
I0608 12:08:21.765405       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:gggggggggggggggg:eeeeeeeeeeeeeeee:1
I0608 12:08:21.864331       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:hhhhhhhhhhhhhhhh:yyyyyyyyyyyyyyyy:1
I0608 12:08:21.864354       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:yyyyyyyyyyyyyyyy:eeeeeeeeeeeeeeee:1
I0608 12:08:21.864369       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:eeeeeeeeeeeeeeee:cccccccccccccccc:1
I0608 12:08:21.864388       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:cccccccccccccccc:xxxxxxxxxxxxxxxx:1
I0608 12:08:21.913089       8 logger.go:32] Reporting span xxxxxxxxxxxxxxxx:xxxxxxxxxxxxxxxx:0:1

Related Issue(s)

Closes #8288

Checklist

• Tests were added or are not required - n/a
• Documentation was added or is not required

Deployment Notes

[doeg]

Caveat that I'm familiar with neither Jaeger nor Datadog, but this looks good to me!