startSpan should take tracepoint location

[ktoso]

resolves #67

This allows the implementation of ad-hoc tracerbullets and configurable per "trace location" tracers.

Here's the gist of it:

        tracer.enableTracepoint(fileID: fileID, line: line)
        // Imagine this is set via some "ops command", e.g. `<control> <pid, or ssh or something> trace enable Sample.swift:1234`
        // Effectively enabling tracepoints is similar to tracer bullets, tho bullets are generally "one off",
        // but here we could attach a trace-rate, so e.g.: control `<pid> trace enable Sample:1234 .2` to set 20% sampling rate etc.

        tracer.withSpan("dont") { span in
            // don't capture this span...
        }
        tracer.withSpan("yes", file: #fileID, line: 7777) { span in
            // do capture this span, and all child spans of it!
            tracer.withSpan("yes-inner") { inner in
                // since the parent of this span was captured, this shall be captured as well
            }
        }

or even better:

tracer.enableTracepoint(function: "example()")

[ktoso]

Paging @weissi - the idea that we talked about yesterday. this would enable various location sensitive configuration or whatever one would want to do in a tracer

[weissi]

why not the new Clock/Duration/... API?

[ktoso]

Yeah just noticed this too tbh...

I'll need to think about this, it'd force users to require 5.7+ so we may not want that... we'd want vapor to adopt without having to suddenly bump their required swift versions. Probably we can add an public API accepting Instant but I guess implementors may have to implement against dispatch time or something else... to keep the swift requirement low :-/

I'll think about it more and ticketify. AHC we'd also not want to force to require 5.7 suddenly 🤔

[ktoso]

We can add an overload though that delegates to the dispatch time one 👍

[ktoso]

I'll add overload in separate PR

[ktoso]

#71

[ktoso]

@swift-server-bot test this please

[slashmo]

LGTM

[slashmo]

Good simplification 👍

[slashmo]

I'm very much looking forward to a time where we can start to only support #fileID in server-side Swift projects 😬

[ktoso]

Tbh NIO requires 5.5+ already, so most projects can. I'm not even sure we should keep trying with 5.1+ here... but for now we do.