Run the guacgql HTTP server on only one port

[nchelluri]

Description of the PR

• Fixes [bug] HTTP address of metrics endpoints clash  #1676

Currently, if I check out main and run go run ./cmd/guacgql --gql-debug, then I will be able to access / (playground), /query (GQL server), and /metrics on both ports 8080 and 9091 of localhost. I don't think that this makes sense so I have changed the behavior in this PR so that all of these paths are served off of just one port.

To test this change, run each of the following commands in a terminal tab:

1. docker run -p 4222:4222 -p 8222:8222 nats -m 8222 -js
2. go run ./cmd/guacgql --gql-debug
3. go run ./cmd/guaccsub
4. go run ./cmd/guaccollect deps_dev

Then you will be able to access the GraphQL playground on http://localhost:8080, the GraphQL server metrics endpoint on http://localhost:8080/metrics, and the guaccollect deps.dev collector metrics on http://localhost:9091/metrics.

In addition, you may add a --gql-listen-port nnn to the second command and see the port for the GraphQL URLs change to nnn, and/or add a --prometheus-port mmm to the fourth command and see the port for the collector metrics change to mmm.

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If OpenAPI spec is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[jeffmendoza]

Yes, makes more sense to have a /metrics path on the same port.

[nchelluri]

Yes, makes more sense to have a /metrics path on the same port.

One note: it was actually on the same port before; http.DefaultServeMux was being served off of two separate ports (both of which had the / - the playground - and /metrics). I tried to clarify in the commit msg.