Decoding grpc-web-text

I'm having trouble decoding the following grpc-web-text response message, referencing the techniques and documents described in #634.

```
AAAAADMKMQoKQ2xvdWRmbGFyZQoHRGlzY29yZAoGR2l0SHViCgpHaXRIdWIgQVBJCgZHb29nbGU=AAAAABMSEQoHRGlzY29yZBIGEIDfoZ8BAAAAABISEAoGR29vZ2xlEgYQwNrkngE=AAAAABYSFAoKQ2xvdWRmbGFyZRIGEIDfoZ8BAAAAABYSFAoKR2l0SHViIEFQSRIGEICBt7ABAAAAAA0SCwoGR2l0SHViGIJWgAAAABBncnBjLXN0YXR1czogMA0K
```

I can identify the first message as encoded proto data just fine.

```
00
00 00 00 33
0a 31 0a 0a 43 6c 6f 75 64 66 6c 61 72 65 0a 07
44 69 73 63 6f 72 64 0a 06 47 69 74 48 75 62 0a
0a 47 69 74 48 75 62 20 41 50 49 0a 06 47 6f 6f
67 6c 65
```

The next part APPEARS to be a simple 4-byte data message, but by my reckoning, it's not a valid protobuf message.
```
00
00 00 00 04
c4 84 42 81
```

According to the spec documents, if I'm reading it right, next I should expect another marker, which should be either 00, 01, 80, or 81. Instead, I get D1.

```
d1 1a 5c d8 db dc 99 04 81 84 20 37 e8 67 c0 40
00 00 00 04 84 84 02 81 91 db db d9 db 19 44 81
84 30 36 b9 27 80 40 00 00 00 01 61 21 40 a0 a4
36 c6 f7 56 46 66 c6 17 26 51 20 61 08 0d fa 19
f0 10 00 00 00 01 61 21 40 a0 a4 76 97 44 87 56
22 04 15 04 91 20 61 08 08 1b 7b 00 10 00 00 00
00 d1 20 b0 a0 64 76 97 44 87 56 21 88 25 68 00
00 00 01 06 77 27 06 32 d7 37 46 17 47 57 33 a2
03 00 d0
```

Clearly, I'm missing something, because the message actually WORKS completely fine. However, what I'm seeing is that this endpoint works in local development, but somehow causes client-side exceptions in the deployed production application, so I got caught on a tangent of trying to figure out how to actually decode these messages, and learn more about what's really going on.

An actual failing message in production is...

```
QUFBQUFETUtNUW9LUTJ4dmRXUm1iR0Z5WlFvSFJHbHpZMjl5WkFvR1IybDBTSFZpQ2dwSGFYUklkV0lnUVZCSkNnWkhiMjluYkdVPUFBQUFBQlFTRWdvS1EyeHZkV1JtYkdGeVpSSUVFTUNFUFE9PUFBQUFBQkFTRGdvR1IyOXZaMnhsRWdRUXdJUTlBQUFBQUJFU0R3b0dSMmwwU0hWaUVnVVF3TldIQ1E9PUFBQUFBQlVTRXdvS1IybDBTSFZpSUVGUVNSSUZFTURWaHdrPUFBQUFBQkVTRHdvSFJHbHpZMjl5WkJJRUVJQ0plZz09Z0FBQUFCQm5jbkJqTFhOMFlYUjFjem9nTUEwSw==
```

I'm guessing this is somehow compressed?