Capture Wire Protocol design goals (#21)

This is a follow up from Spec SIG meeting on Jul 18, 2019, recapping
the topics that I mentioned in the meeting.

The document will help us design the right wire protocol.

specification/proto-design-goals.md

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+# Design Goals for OpenTelemetry Wire Protocol
+
+We want to design a telemetry data exchange protocol that has the following characteristics:
+
+- Be suitable for use between all of the following node types: instrumented applications, telemetry backends, local agents, stand-alone collectors/forwarders.
+
+- Have high reliability of data delivery and clear visibility when the data cannot be delivered.
+
+- Have low CPU usage for serialization and deserialization.
+
+- Impose minimal pressure on memory manager, including pass-through scenarios, where deserialized data is short-lived and must be serialized as-is shortly after and where such short-lived data is created and discarded at high frequency (think telemetry data forwarders).
+
+- Support ability to efficiently modify deserialized data and serialize again to pass further. This is related but slightly different from the previous requirement.
+
+- Ensure high throughput (within the available bandwidth) in high latency networks (e.g. scenarios where telemetry source and the backend are separated by high latency network).
+
+- Allow backpressure signalling.
+
+- Be load-balancer friendly (do not hinder re-balancing).