adds a CompositeSpanExporter

[nelsitoPuglisi]

Goal

Creates a CompositeSpanExporter to add new instances of SpanExporter at runtime.

I considered using the io.opentelemetry.SpanExporter.composite() method (link here). Still, it can't be modified at runtime, so it's not helpful to the purpose of allowing a client app to add a SpanExporter.

[nelsitoPuglisi]

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Current dependencies on/for this PR:

• master
  • PR add Resource with service.name and service.version #375
    • PR addSpanExporter api #377
    • PR adds a CompositeSpanExporter #376 👈

This stack of pull requests is managed by Graphite.

[nelsitoPuglisi]

I have considered moving the exporter instance inside our OpenTelemetrySDK wrapper, but the injection of the EmbraceSpanProcessor didn't allow me to.

[nelsitoPuglisi]

CompositeSpanExporterTest was added in the #377 😬

[bidetofevil]

Looks fine for the demo, but ideally we can get rid of our own composite. Creating it before we start the SDK would mean that we never have a gap between when the SDK is created and when the customer adds the exporter.

[bidetofevil]

This could be internal

[bidetofevil]

Hmm. The reason you can't use the default composite one is because we are initializing this too early, before the app can add the exporter instance? If this were happening later, I can imagine a list of exporters passed into InitModuleImpl and we just call SpanExporter.composite() here

[nelsitoPuglisi]

Exactly. We are exporting the emb-startup-moment span, though.

[fractalwrench]

LGTM pending resolution of Hanson's comments