web_project_client = Table('web_project_client', Base.metadata,
    Column('web_project_id', Integer, ForeignKey('web_projects.id', ondelete='CASCADE', onupdate='CASCADE')),
    Column('client_id', Integer, ForeignKey('clients.id', ondelete='CASCADE', onupdate='CASCADE')),
)

class WebProject(Base):
    __tablename__ = 'web_projects'
    id = Column(Integer, primary_key=True)
    metrics = relationship("MetricsJSON", backref="web_project")
    ...
    clients = relationship('Client',
        secondary=web_project_client,
        backref=backref('web_projects', lazy='dynamic'),
        lazy='dynamic'
    )
    ...

class Client(Base):
    __tablename__ = 'clients'
    id = Column(Integer, primary_key=True)
    ...
    metrics = relationship("MetricsJSON", backref="client", lazy='dynamic')
    ...

class MetricsJSON(Base):
    __tablename__ = 'metrics_json'
    id = Column(Integer, primary_key = True)
    metrics = Column(JSONB)

    timestamp = Column(DateTime, default=datetime.utcnow())

    web_project_id = Column(Integer, ForeignKey('web_projects.id'))
    client_id = Column(Integer, ForeignKey('clients.id'))
    ...