Custom metadata not included in HTTP headers when using storage emulator

[cortadocodes]

Describe the bug

When custom metadata is added to a cloud object and you access it via an HTTP GET request, each key-value pair should be included in the response headers with the prefix x-goog-meta-<key>. These key-value pairs are missing from the response headers when the storage emulator is used.

For example, if I add this metadata to a cloud object:

{"some": "metadata"}

then there should be a header in the response that looks like this:

{"x-goog-meta-some": "metadata"}

Instead, the only headers included in the response are:

{
    'Server': 'BaseHTTP/0.6 Python/3.9.13', 
    'Date': 'Wed, 29 Jun 2022 15:59:33 GMT', 
    'X-Goog-Hash': 'crc32c=pdRKBQ==,md5=vrakOt+5UOxvgs7tGb7uIQ==', 
    'Content-type': 'text/plain',
    'Content-Length': '10'
}

To Reproduce

With the emulator running, execute the following python script:

import requests        
from google.cloud import storage


client = storage.Client()
client.create_bucket("my-bucket")

blob = client.get_bucket("my-bucket").blob(blob_name="my_file.txt")
blob.upload_from_string("blah blah blah")
blob.metadata = {"some": "metadata"}
blob.patch()

url = f"{os.environ['STORAGE_EMULATOR_HOST']}/my-bucket/my_file.txt"
response = requests.get(url)
assert response.headers["x-goog-meta-some"] == "metadata"

This will raise:

KeyError: 'x-goog-meta-some'

Expected behavior

Custom metadata (key-value pairs) should appear in the headers as key-value pairs like:

x-goog-meta-<key>: <value>

System (please complete the following information)

• OS version: MacOS Monterey 12.4
• Python version: 3.9.13
• gcp-storage-emulator version: 2022.06.11

Additional context

I came across this while creating signed URLs to cloud objects and attempting to get their metadata, but this problem occurs for all URLs to cloud objects.