Try:

prometheus:
   container_name: prometheus
   image: "prom/prometheus:latest"
   user: "1000"
   networks:
     - observability
   ports:
     - "9090:9090"
   volumes:
     - ./prometheus:/etc/prometheus
     - /data/prometheus:/var/lib/prometheus
   command:
     - --config.file=/etc/prometheus/prometheus.yml
     - --storage.tsdb.path=/var/lib/prometheus
     - --web.enable-admin-api
   restart: unless-stopped

The command property takes a YAML list of values.

Each value must be indicated using a hyphen (- {value}).

The list values are double-hyphen flags (--web.enable-admin-api).

Alternatively, you can use the JSON variant which I've spread over multiple lines but can also be presented as a single line:

command: [
  "--config.file=...",
  "--storage.tsdb.path=...",
  "--web.enable-admin-api"
]