Opt-in core-to-REST-server instrumentation

[johnkerl]

Provides crucial information on core-to-REST-server HTTP operations. This is essential for analyzing and minimizing remote-request latencies. An indispensable counterpart to Jaeger tracing, while Jaeger tracing isn't enough to give us a full picture on all interactions in all contexts. Having this in-core will make these performance numbers readily accessible where they're most needed, namely, in notebook environments.

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TYPE: IMPROVEMENT DESC: Opt-in core-to-REST-server instrumentation

[johnkerl]

@ypatia is there anything else to do on this PR? If not can we approve it?

[ypatia]

@ypatia is there anything else to do on this PR? If not can we approve it?

It's ok for me, but let's have an approval from either @Shelnutt2 or @ihnorton before we merge

[ypatia]

As per @Shelnutt2's initial review we have:

• Removed flush()
• Removed the config variable all along as it was only needed to disable flush() if that would bring perf degradation. Now only trace level logging level is needed.
• Replaced LOG_TRACE() macro w/ using the structured logger_ member of Curl class to give finer debug info on what query is executing.

[johnkerl]

🚀