Check network connectivity before attempting HTTP requests in export

[fractalwrench]

Goal

On mobile platforms there should be a configurable option to check whether a network connection is available. If none is available, then export involving HTTP requests should not be attempted and telemetry should be queued on disk.

Related to #69

[bidetofevil]

An export may not necessary rely on a network, so maybe this should just be part of the persistence/delivery layer assures exports are attempted and retried in an appropriate cadence if the initial attempt fails, i.e. an exporter that requires a network will be aware that there's no network and won't attempt to export in such a case?

[fractalwrench]

Good point. I've tweaked the title and description a bit to reflect this.

[ArielDemarco]

I agree with @bidetofevil; this is probably better handled as part of the behavior of a specific exporter (e.g., the HTTP exporter).

FWIW, in OpenTelemetry-Swift we have a Persistence Exporter (decorator) that addresses this kind of use case. Quoting the docs:

An example use case is mobile apps that operate while the device has no network connectivity. With the Persistence Exporter decorating the actual exporters used by the app, telemetry can be collected while the device is offline. Later - possibly after the app is terminated and relaunched - when network connectivity is back the collected telemetry data can be picked up from the disk and exported.