Envoy metrics receiver plugin protos (#2304)

* Add envoy metrics service protos

This patch imports protos from envoyproxy/data-plane-api and
prometheus/client_model to enable metrics service service feature in
SkyWalking OAP.

Signed-off-by: Dhi Aurrahman <dio@tetrate.io>

* Add Envoy related docs

Signed-off-by: Dhi Aurrahman <dio@tetrate.io>

* Update the LICENSE to include new protos license

From:
- envoyproxy/data-plane-api
- prometheus/client_model

Signed-off-by: Dhi Aurrahman <dio@tetrate.io>

* Add a complete config.yaml example

Signed-off-by: Dhi Aurrahman <dio@tetrate.io>

* Add note on Envoy dynamic config

Signed-off-by: Dhi Aurrahman <dio@tetrate.io>

* Add protoc-gen-validate license

Signed-off-by: Dhi Aurrahman <dio@tetrate.io>

* Add samples

Signed-off-by: Dhi Aurrahman <dio@tetrate.io>

* Make CI and license right.

* Update README.md

@dhi typo :P

* Update README.md

Another typo :P

* Delete metrics.json

metrics.json is included in identify.json and not be linked in document.