Previousy, transport telemetry was recorded by emitting Events from an
IO instance to an aggegator. This requires that each update take a
global telemetry lock, and is an impediment to richer telemetry.
This change removes the transport event types so that the Event and
Record types are left only to represent HTTP telemetry. Now, the
transport's IO type holds a reference to a shared `Metrics` structure.
As the transport is used, metric values are updated immediately.
A lock on the transport _registry_ is taken whenever a new transport is
opened/accepted and when metrics are reported. Each transport class's
metrics are now shared & locked independently, so it's possible for a
transport to update its metrics while the registry is being manipulated.
This has one functional change: the `tcp_read_bytes_total` and
`tcp_write_bytes_total` counters are now updated instantaneously.
Previously these values were only incremented on transport close, which
is misleading, especially for long-lived connections.
With this change, all transport-related telemetry logic lives in
`telemetry::transport`.
Oliver Gould