Instana exporter implementation.

[kothiga]

This PR is to contribute an OpenTelemetry exporter which facilitates conversion and transmission of spans to Instana.

I have also opened two other PRs for documentation related to this work

• Instana exporter documentation. opentelemetry-specification#4346
• Instana exporter documentation. opentelemetry.io#5834

I hope that I have added everything correctly for the read-only git subtree split.

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[brettmc]

Hi @kothiga
The opentelemetry-php repository is for our implementation of the opentelemetry specification, and none of this stuff is in the spec.
Taking a quick look through instana's docs, I see that Instana can seamlessly integrate with OpenTelemetry. Native OTLP support for ingesting telemetry is the direction most if not all APM vendors are moving in, and that's what we're all about here.
So, if Instana already natively supports the OTLP protocol, why is this PR required? And if it is required for some use-cases, wouldn't it be better done in the opentelemetry collector, so that it would work for all languages?