Google-cloud-python: Add `client_info` to constructors for manual clients

[tswast]

The client_info constructor argument is the standard argument for changing user agent headers in GAPIC clients. Adding a user_agent prefix identifying an app or tool is required for many partners who use Google APIs.

Handwritten clients should provide a similar client_info argument to the client constructor. See how this was done in BigQuery at https://github.com/googleapis/google-cloud-python/pull/7806.

Clients that need client_info:

• [x] BigQuery (https://github.com/googleapis/google-cloud-python/pull/7806)
• [x] [Bigtable](https://github.com/googleapis/google-cloud-python/blob/af3df9cc2c520ef48870e124b65849c37966c43e/bigtable/google/cloud/bigtable/client.py#L76) (#7876)
• [x] [Datastore](https://github.com/googleapis/google-cloud-python/blob/92465cbc4d9c0ba251838e9cd17f61d14b470e04/datastore/google/cloud/datastore/client.py#L165)
• [x] [DNS](https://github.com/googleapis/google-cloud-python/blob/ff05ac3a8fa58a696584bada6ac89eb0a81e9856/dns/google/cloud/dns/client.py#L24) (#7869)
• [x] [Error Reporting](https://github.com/googleapis/google-cloud-python/blob/92465cbc4d9c0ba251838e9cd17f61d14b470e04/error_reporting/google/cloud/error_reporting/client.py#L84) (#7903)
• [x] [Firestore](https://github.com/googleapis/google-cloud-python/blob/894243931e41e1e9e71762d4cabd2dbe7c39b6af/firestore/google/cloud/firestore_v1/client.py#L51) (and Firestore Beta) (#7877, V1 only, see below)
• [x] [Logging](https://github.com/googleapis/google-cloud-python/blob/aee5ceca1561e7f2c92a71b7be026851c5cb5d41/logging/google/cloud/logging/client.py#L59) (#7874, #7901)
• [x] Pub/Sub (Publisher, Subscriber) (done via **kwargs)
• [x] [Resource Manager](https://github.com/googleapis/google-cloud-python/blob/ff05ac3a8fa58a696584bada6ac89eb0a81e9856/resource_manager/google/cloud/resource_manager/client.py#L26) (#7870)
• [x] [Runtime Config](https://github.com/googleapis/google-cloud-python/blob/9e804dfb2fa1ca4a29e861d88600f924906776d6/runtimeconfig/google/cloud/runtimeconfig/client.py#L24) (#7871)
• [x] [Spanner](https://github.com/googleapis/google-cloud-python/blob/ff05ac3a8fa58a696584bada6ac89eb0a81e9856/spanner/google/cloud/spanner_v1/client.py#L78) - Note: Spanner has a user_agent argument already, but it's currently a no-op. Should mark that as deprecated. (#7878)
• [x] [Storage](https://github.com/googleapis/google-cloud-python/blob/ff05ac3a8fa58a696584bada6ac89eb0a81e9856/storage/google/cloud/storage/client.py#L32) (#7872)
• [x] [Trace](https://github.com/googleapis/google-cloud-python/blob/c0abaa4d6860bcdd621d00d0e0bad94237b964b5/trace/google/cloud/trace/client.py#L21) and Trace V1 (#7899)
• [x] [Translate](https://github.com/googleapis/google-cloud-python/blob/9e804dfb2fa1ca4a29e861d88600f924906776d6/translate/google/cloud/translate_v2/client.py#L35) (#7873, no GRPC version needed, because v3beta1 is pure-autogen)

I've heard requests for this feature in the Storage client, so I'd prioritize that one above the others, but we do need this feature across all clients.

[tseaver]

My current plan:

• Hoist @tswast's work from #7806 up into the google.cloud._http.JSONConnection base class. (#7849)
• Deprecate the USER_AGENT and _EXTRA_HEADERS class-scope constants in that class, in favor of new instance properties, user_agent / extra_headers. Replace the deprecated class-scope names with properties which emit warnings when assigned. (#7849)
• Document the client_info parameter in all classes which derive from JSONConnection.
• Add client_info to the constructor of those clients which use connections based on JSONConnection, and pass it through to the connections (storage, dns, logging, resource_manager, runtimeconnection, translate).
• For clients which do not use JSONConnection, add client_info to their constructors and forward it to their GAPIC clients.

[tseaver]

@tswast One thing I've noticed while deprecating the _EXTRA_HEADERS variable: you bend over backwards to allow self._EXTRA_HEADERS["foo"] = "bar". I think I'd rather avoid supporting that, and instead just allow assignment to it. WDYT?

[tswast]

I think I'd rather avoid supporting that, and instead just allow assignment to it. WDYT?

Seems reasonable, since we're making a new property name now. I bent over backwards because we had some tests that did key assignment and I wanted to maintain compatibility.

[tseaver]

@tswast I just realized that since we've hoisted the logic for adding the X-Goog-API-Client header into the base class, we can avoid jamming it into the extra_headers return value, which makes allowing assignment into it simple.

[tseaver]

@tswast I think we should leave this feature out of the firestore_v1beta1 client object: the Firestore team really doesn't want to encourage new users for that version of the API.

[tswast]

Thanks @tseaver ! The REST clients LGTM. A few questions on the gapic clients.

[tswast]

🎉 Thanks! 🎉

Are we following-up with a bunch of releases next (or at least for api_core)?