Add http.active.requests and http.duration metrics

[attilakreiner]

Description

This change adds these metrics:

• http.active.requests to track the number of currently active http requests (gauge)
• http.duration to track to track the duration of http requests (histogram)

Fixes #111

[jfallows]

i think we need to adjust the line:

https://github.com/aklivity/zilla/blob/feature/telemetry-metrics/runtime/engine/src/main/java/io/aklivity/zilla/runtime/engine/internal/registry/DispatchAgent.java#L1328

To use the following instead:

final MessageConsumer replyTo = supplyReplyTo(initialId).andThen(sentMetricHandler);

because these lines fill in the timestamp during writing of frames, such as END frame.

https://github.com/aklivity/zilla/blob/feature/telemetry-metrics/runtime/engine/src/main/java/io/aklivity/zilla/runtime/engine/internal/stream/Target.java#L134

so that needs to be done first, before metrics try to observe the timestamp value, otherwise it will show timestamp value 0.