Support additional http specific metrics

[jfallows]

Note: requires design #109.

The following http metrics should be contributed to the Zilla engine's awareness of available metrics via the telemetry-http component, consistent with other pluggable aspects of the runtime.

• http.duration: histogram
• http.request_size: histogram
• http.response_size: histogram
• http.active_requests: updown

Note that the built-in counters for Zilla streams are all counters, so this will introduce new metrics types for updown and histogram.

The nature of the duration metric is also more stateful as there is a need to keep track of the request start timestamp and the later of the request and response end timestamps to capture the full duration of any request-response interaction.

Note: we need to validate that zilla metrics is reporting these new metrics types correctly as well.