Add stats.Handler support to inprocgrpc.Channel (#83)

Implements support for the google.golang.org/grpc/stats.Handler API in
the in-process channel, enabling instrumentation with OpenTelemetry and
other observability tools that use stats handlers instead of the
now-deprecated interceptor-based APIs.

related #73

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Signed-off-by: Roman Dmytrenko <rdmytrenko@gmail.com>

protoc plugin supports proto3 optional (#63)

oof, major bug: infinite loop when trying to intercept an already-int…

…ercepted channel (#61)

fix go.mod for Go 1.13 (#34)

inprocgrpc should support dynamic messages (#32)