Add datadog tracing support

[erickhun]

Append trace_id and span_id to the logs.
We use the mixin() function of pino to append traces to the logs.

Pre-Requirement:

The service using the lib will need to also have the dd-trace library installed. If not, nothing will be appended to the logs.

How to initialise the tracer in a service:

import tracer from "dd-trace";
tracer.init({
    hostname: "dd-agent-hostname",
    logInjection: false
});

# then BuffLog will automatically detect instance of `tracer` and inject the `trace_id`/`span_id`

Results:

# outside a request: 
{"level":35,"time":1582527777954,"message":"hello notice","v":1}
{"level":40,"time":1582527777955,"message":"hello warning","v":1}
{"level":50,"time":1582527777955,"message":"hello error","v":1}
{"level":60,"time":1582527777955,"message":"hello critical","v":1}

# during a request: 
Server is listening on port 4000
{"level":35,"time":1582527789115,"dd":{"trace_id":"4471443652644886255","span_id":"8148478562811089665"},"message":"Log via endpoint","v":1}
{"level":35,"time":1582527789116,"dd":{"trace_id":"4471443652644886255","span_id":"8148478562811089665"},"message":"hello notice","v":1}
{"level":40,"time":1582527789117,"dd":{"trace_id":"4471443652644886255","span_id":"8148478562811089665"},"message":"hello warning","v":1}
{"level":50,"time":1582527789117,"dd":{"trace_id":"4471443652644886255","span_id":"8148478562811089665"},"message":"hello error","v":1}
{"level":60,"time":1582527789117,"dd":{"trace_id":"4471443652644886255","span_id":"8148478562811089665"},"message":"hello critical","v":1}

cc @esclapes @colinscape if you have any comments :) ( I also tried to add tests with #3 , but I couldn't figure out yet how to properly test what's sent to stdout )

[esclapes]

Hey @erickhun,

Curious to know what differences you found between using logInjection: true which supports pino and doing the injection manually like we are doing in this PR. Is the injected data different?

[erickhun]

@esclapes good one, i've tried to add the traceID with switching the logInjection to true as you said, but it didn't work out of the box 🤔. It's why i've ended up with the manual tracing.

I'm not too sure how we can make it detect by the dd-trace plugin out of the box. but if you have any idea to make that work that'd be awesome.

[esclapes]

@erickhun 🤔 my hunch is that it's an issue with the order of the imports. I believe the .init call needs to happen before anything else is imported so that dd-trace has time to monkey patch the supported libraries.

We usually place all the init code in a separate file and then import that file at the top of the index. This is a recent example

[erickhun]

@esclapes oh thanks! i'll try this in an other branch. Sorry if that might look obvious, still trying to get up to speed with js 😅

[erickhun]

@esclapes implemented in #8

It also worked if we don't do it in an other file 🤔. I think I've perhaps not switched the autoinject parameter properly 🙈

Thanks for your feedbacks, definitely make it easier to use!

[esclapes]

Glad to know that it works! It feels nice to use the dd-trace feature for this, as it will probably be updated with newer features and bug fixes that may not be obvious to us.

💯