implement gRPC rest gateway

[lampajr]

Description

Fixes opendatahub-io/model-registry#32

Automate the generation of a gRPC rest gateway proxy in front of the actual gRPC server.

Startup the gateway:

$ ./model-registry gateway

or

$ make run/gateway

NOTE: right now I setup a 10s timeout on gRPC gateway, this means that if you start it up and the gRPC is not running yet it will fail and exit after 10s

How Has This Been Tested?

In order to test this I created a new python script test/python/test_mlmetadata_rest.py that aims to create some ArtifactType and an Artifact instance dealing with the REST interface instead of the gRPC one.

Steps to run the tests:

# in one terminal
$ make run/server

# in another terminal
$ make run/gateway

# in third terminal
$ python test/python/test_mlmetadata_rest.py

Merge criteria:

• The commits and have meaningful messages; the author will squash them after approval or will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[rareddy]

I there a reason why we want this to be a different process from the main gRPC server? Can not wire up such that it is a single process?

[dhirajsb]

@lampajr this looks quite good 👍
I'll review it in more detail tomorrow.

[lampajr]

I there a reason why we want this to be a different process from the main gRPC server? Can not wire up such that it is a single process?

Initially because as per my understanding the gRPC gateway needs to dial a connection to the gRPC server (that is why I thought having different processes would have been a better approach) - on the other hand I am evaluating if we can sort this out having a single process as well.

[dhirajsb]

@lampajr let's hold on to this PR for now while we investigate the envoy proxy that google's ml-metadata server uses.
@tarilabs noticed that it might be exposing an openapi v3 service, which would be preferable.

[lampajr]

Closing as not needed for now.