Bandit-only fork: drop Plug.Cowboy, add bandit_opts

[asib]

Summary

Builds on upstream #266 (Bandit support), then simplifies the metrics server to Bandit-only and exposes transport-level configuration so callers can pass through Thousand Island options.

• Cherry-picks the three commits from feat(metrics_server): Add Bandit support akoutmos/prom_ex#266.
• Collapses the compile-time cond do … end in lib/prom_ex.ex to a single Bandit-only server_spec/5. Drops the Plug.Cowboy branch and the cowboy_opts / pool_size config keys.
• Promotes :bandit and :plug to required deps; drops :plug_cowboy.
• Adds a bandit_opts: keyword in metrics_server config that's spliced into Bandit.child_spec/1, with defensive Keyword.drop for :scheme/:plug/:port.
• Tests for config parsing and child-spec injection (test/prom_ex/config_test.exs, test/prom_ex/metrics_server/startup_test.exs).

Why

fly_deploy's blue/green mode injects `reuseport: true, reuseaddr: true` into the Phoenix Endpoint's transport at peer-boot so old and new peers can both bind the same port during cutover. We want the same overlap-tolerance for the standalone metrics listener — without it, the new peer fails `:eaddrinuse` during the brief overlap.

Example downstream config (sprites-api):

```elixir
config :sprites, Sprites.PromEx,
metrics_server: [
port: 9568,
path: "/metrics",
protocol: :http,
auth_strategy: :none,
bandit_opts: [
thousand_island_options: [
transport_options: [reuseport: true, reuseaddr: true]
]
]
]
```

Test plan

• `mix deps.get && MIX_ENV=test mix compile --warnings-as-errors` — clean.
• `mix test test/prom_ex/config_test.exs test/prom_ex/metrics_server/` — 22/22 pass (includes 6 new tests for `bandit_opts`).
• Full suite (excluding two phoenix_test files pre-broken on upstream master by an Elixir 1.18 macro escaping issue): 4 doctests + 116 tests, 0 failures.
• Manual verification: two `Bandit.start_link` calls with this `bandit_opts` config bind 9568 simultaneously; a second listener without the config fails `:eaddrinuse`.

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