Re: POC: Extension for adding distributed tracing - pg_tracing

>Here's a new patch with changes from the previous discussion: 

>- I'm now directly storing nanoseconds duration in the span instead of the instr_time. Using the instr_time macros was a bit awkward as the durations I generate don't necessarily have a starting and ending instr_time. 

>Moving to straight nanoseconds made things clearer from my point of view.

Cool. It could be easily casted to ms for the user.

>- I've added an additional sample rate pg_tracing.sample_rate (on top of the pg_tracing.caller_sample_rate). This one will allow queries to be sampled even without trace information propagated from the caller. 

>Setting this sample rate to 1 will basically trace everything. For now, this will only work when going through the post parse hook. I will add support for prepared statements and cached plans for the next patch.

Cool, I've just made the same improvement and wanted to send a patch a bit later after tests.

>- I've improved how parse spans are created. It's a bit challenging to get the start of a parse as there's no pre parse hook or instrumentation around parse so it's only an estimation.

I've also added a query id field to span and made a table and an sql function that flushes spans to this table instead of returning set or records - it is more convenient for the maintenance to query the table.

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