metrics: generate grpc-gateway related files and structs

[odeke-em]

The Trace endpoint allows sending data serialized as JSON to the agent. There are many cases where folks want to also serialize metrics coming say from a front-end or a spot that doesn't use grpc nor protobufs so let's add support for metrics too.

[odeke-em]

/cc @draffensperger

[odeke-em]

I believe it would entail adding

# This is an API configuration to generate an HTTP/JSON -> gRPC gateway for the
# OpenCensus service using github.com/grpc-ecosystem/grpc-gateway.
type: google.api.Service
config_version: 3
http:
 rules:
 - selector: opencensus.proto.agent.metrics.v1.MetricsService.Export
   post: /v1/metrics
   body: "*"

to
src/opencensus/proto/agent/metrics/v1/metrics_service_http.yaml
but that'll also require regeneration of the protos of which my protoc compiler has historically radically deviated from @songy23's so kindly also paging @songy23, otherwise I'd send a PR.

[draffensperger]

Yes, I like the idea. One big caveat with metrics sent from ephemeral clients like web browsers, internet-of-things devices, or even things like Lambda/Cloud functions:

• It's probably not enough in those cases to just expose a way for them to write arbitrary metrics, and it could even be risky - it could allow one client to overwrite aggregated points
• Instead what we want for those clients is the ability to write points that get aggregated up into cumulative counters and distributions by the service itself. One proof of concept we did as part of a hackathon did that where the browse clients writes a GAUGE and then the service adds it up from all clients to form a distribution / cumulative counter.

Anyway, there isn't harm in adding the gRPC gateway support, which could also help some server-side cases as well, like writing metrics from a bash script or simple Python script where they don't want to bring in gRPC or any libraries.