Naming conventions in metadata sequences are ignored

Describe the bug

Metadata sequences were implemented in #38, only for auto generation, in 7d5f6eabeccb9328c1c21dd0cdf9ff1f2f85836d.

The SchemaObjects.metadata() is older than the sequences implementation and don't check if it is a list. https://github.com/sqlalchemy/alembic/blob/main/alembic/operations/schemaobj.py#L193-L202

Expected behavior

Naming conventions should be respected with metadata sequences.

To Reproduce
Please try to provide a Minimal, Complete, and Verifiable example, with the migration script and/or the SQLAlchemy tables or models involved.
See also Reporting Bugs on the website.

from sqlalchemy import MetaData
from alembic.config import Config
from alembic.operations.schemaobj import SchemaObjects
from alembic.runtime.environment import EnvironmentContext
from alembic.script import ScriptDirectory

convention = {
    "ix": "foo_ix_%(table_name)s_%(column_0_name)s",
}

context = EnvironmentContext(Config(), ScriptDirectory("."))
context.configure(
    dialect_name="postgresql",
    target_metadata=[
        MetaData(naming_convention=convention),
        MetaData(),
    ],
)
# This is unexpectedly the default naming convention `immutabledict({'ix': 'ix_%(column_0_label)s'})`
print(SchemaObjects(context.get_context()).metadata().naming_convention)

context = EnvironmentContext(Config(), ScriptDirectory("."))
context.configure(
    dialect_name="postgresql",
    target_metadata=MetaData(naming_convention=convention),
)
# It works as expected
print(SchemaObjects(context.get_context()).metadata().naming_convention)

Error

There is no error, but it doesn't use my conventions.

Versions.

• OS:
• Python:
• Alembic:
• SQLAlchemy:
• Database:
• DBAPI:

Additional context

Have a nice day!

Start with alembic/operations/schemaobj.py around lines 193-202 and run the reproduction using SchemaObjects.metadata() with a list of MetaData objects. Compare the list and single-MetaData cases; done means the configured naming convention is preserved when metadata sequences are used.