support grpc reflection

[mlindes]

This enables support for gRPC reflection.

When running otel-cli server tui in one terminal, from another, use grpCurl to query the otel-cli endpoint at localhost:4317

» grpcurl -plaintext localhost:4317 list
grpc.reflection.v1.ServerReflection
grpc.reflection.v1alpha.ServerReflection
opentelemetry.proto.collector.trace.v1.TraceService

without this, an error is received:

Failed to dial target host "localhost:4317": dial tcp [::1]:4317: connect: connection refused