Extract byte size from proto-plus messages

[aabmass]

Fixes #969

Recommendation for getting the byte size here: googleapis/proto-plus-python#163

[aabmass]

Just doing some testing on this before merging

[aabmass]

Did some manual testing, working as expected with proto-plus versions of a GCP client lib (used google-cloud-secret-manager==2.1.0):

import time

from google.cloud.secretmanager_v1.types.service import (
    AccessSecretVersionRequest,
)
from opencensus.trace.tracer import Tracer
from opencensus.trace.samplers import AlwaysOnSampler
from opencensus.trace import config_integration
from opencensus.ext.stackdriver.trace_exporter import StackdriverExporter
from opencensus.common.transports.async_ import AsyncTransport
from functools import partial
from google.cloud.secretmanager_v1.services.secret_manager_service import (
    SecretManagerServiceClient,
)


config_integration.trace_integrations(["google_cloud_clientlibs"])

tracer = Tracer(
    exporter=StackdriverExporter(transport=partial(AsyncTransport, wait_period=5.0)),
    sampler=AlwaysOnSampler(),
)


def main() -> None:
    secret_client = SecretManagerServiceClient()

    secret = secret_client.access_secret_version(
        AccessSecretVersionRequest(
            name="projects/<projectid>/secrets/fake-secret/versions/1"
        )
    ).payload.data.decode()
    print("Got secret: {}".format(secret))
    time.sleep(6)


if __name__ == "__main__":
    main()

And I see the byte size annotations in Cloud trace: