add otel push exporter

[mellowagain]

adds new features:

• otel-push-exporter (parent feature, enabled automatically if one of the features below are)
• protocols: otel-push-exporter-http and otel-push-exporter-grpc
• runtimes: otel-push-exporter-tokio, otel-push-exporter-tokio-current-thread and otel-push-exporter-async-std

in the documentation we should advise on using those features if one wants to use the otel push exporter. the exporter can be invoked easily by calling

otel_push_exporter::init_http("url")?;
// or
otel_push_exporter::init_grpc("url")?;

depending on which feature flags are enabled and which protocol one wants to use. the returned OtelMeterProvider has a custom Drop implementation which will ensure that .shutdown is called before the provider is dropped.

[hatchan]

This period is that the same as push interval? If so, then I think we should not use such a low value, not sure what would be a better value 🤔 Maybe something like 30 seconds.

[mellowagain]

im actually thinking maybe we shouldnt set these at all. then otlp exporter would fall back to using the env variables OTEL_METRIC_EXPORT_TIMEOUT and OTEL_METRIC_EXPORT_INTERVAL and if not found fall back to the default value, currently 30 timeout and 60 interval

[mellowagain]

i have done so now

[gagbo]

I forgot about this, using 30 seconds is probably the better default, but also maybe it'll create struggles with the explorer with the build_info metric being looked for only on the last second. Didn't have time to test Brett's fix, maybe that's not an issue anymore

[hatchan]

The only thing that I think we need is some docs about how to use this in the README or autometrics-dev/docs