Add missing data to GCP integration.

[lucas-zimerman]

Based on: https://github.com/getsentry/sentry-python/blob/4c09f3203d6d19789c6fa729a2e46557ad4ea913/sentry_sdk/integrations/gcp.py
Tag

                scope.set_tag("gcp_region", environ.get("FUNCTION_REGION"))

Transaction Name

            transaction = Transaction.continue_from_headers(
                headers, op="serverless.function", name=environ.get("FUNCTION_NAME", "")

Context

        extra["google cloud functions"] = {
            "function_name": environ.get("FUNCTION_NAME"),
            "function_entry_point": environ.get("ENTRY_POINT"),
            "function_identity": environ.get("FUNCTION_IDENTITY"),
            "function_region": environ.get("FUNCTION_REGION"),
            "function_project": environ.get("GCP_PROJECT"),

After looking for some data available on .NET I have found the following ones:

K_REVISION: Release version
FUNCTION_TARGET: namespace + function name
FUNCTION_SIGNATURE_TYPE : http (or event?)
K_SERVICE: the GCP function name
GAE_RUNTIME: dotnet3
I don't think we have the GCP Project name, nor the region nor the other data

[lucas-zimerman]

Implemented on #1286