Re-instate `(Decompressor).DecompressedSize` optimization

This is a) for parity with how gRPC v1.56.3 worked. But also
it showed up as a giant regression in allocations, as we were
now going through `io.Copy` which allocates a temporary
32KiB buffer. Our payloads are often much smaller.

envconfig: default GRPC_ENFORCE_ALPN_ENABLED to false

CRDB at v1.68.0 fails to communicate with CRDB at v1.56.3 due to this
check. This is strange, since CRDB uses gRPC throughout, and in both
v1.56.3 and v1.68.0 uses `tls.NewConfig` which ensures that `NextProtos`
always contains `h2`.

gRPC introduced this check in grpc#7535.

See: cockroachdb/cockroach#136278 (comment)

transport/http2: use HTTP 400 for bad requests instead of 500

Non-servicable HTTP requests from clients previously resulted in an HTTP
500 response, but no error exists within the server; the client simply
sent a malformed request. Detect bad requests and return HTTP 400 Bad
Request instead of HTTP 500 Internal Server Error.

fixes grpc#5802

transport/http2: use HTTP 400 for bad requests instead of 500

Non-servicable HTTP requests from clients previously resulted in an HTTP
500 response, but no error exists within the server; the client simply
sent a malformed request. Detect bad requests and return HTTP 400 Bad
Request instead of HTTP 500 Internal Server Error.

fixes grpc#5802

Cherry-pick observability changes from master to v1.50.x and update v…

…ersion to 1.50.1 (grpc#5722)

* Add binary logger option for client and server (grpc#5675)

* Add binary logger option for client and server

* gcp/observability: implement public preview config syntax, logging schema, and exposed metrics (grpc#5704)

* Fix o11y typo (grpc#5719)

* o11y: Fixed o11y bug (grpc#5720)

* update version to 1.50.1

Change version to 1.50.0 (grpc#5685)

Change version to 1.51.0-dev (grpc#5687)

Change version to 1.33.1

Change version to 1.34.0-dev (grpc#3941)

stalebot: send first ping after 6 days (grpc#3923)