Log improvements

[dmke]

From my TODO list:

• switch to a structured logger (like zap)
• include X-Request-Id generated in service/middleware/requestid.go
• add a log line when adding files
• add --log-level CLI flag to set log level

[dmke]

zap is included since 9447a8d, but I don't like the formatting default console encoding:

2022-03-17T13:40:10.806+0100    INFO    exec/docker_client.go:66        image already present   {"image": "texlive/texlive:latest"}
2022-03-17T13:40:10.806+0100    INFO    service/service.go:80   starting server {"addr": ":2201"}
2022-03-17T13:40:16.904+0100    INFO    tex/document.go:107     adding file     {"request-id": "0767bf4c-a8ee-4832-836f-df6a7d096224", "filename": "chapter/input.tex"}
2022-03-17T13:40:16.905+0100    INFO    tex/document.go:107     adding file     {"request-id": "0767bf4c-a8ee-4832-836f-df6a7d096224", "filename": "input.tex"}
2022-03-17T13:40:16.905+0100    INFO    tex/document.go:151     found \documentclass    {"request-id": "0767bf4c-a8ee-4832-836f-df6a7d096224", "filename": "input.tex"}
2022-03-17T13:40:16.905+0100    INFO    tex/document.go:107     adding file     {"request-id": "0767bf4c-a8ee-4832-836f-df6a7d096224", "filename": "doc.tex"}
2022-03-17T13:40:16.905+0100    DEBUG   exec/docker.go:29       running latexmk {"request-id": "0767bf4c-a8ee-4832-836f-df6a7d096224", "args": ["-cd", "-silent", "-pv-", "-pvc-", "-pdfxe", "input.tex"]}
2022-03-17T13:40:16.961+0100    DEBUG   exec/docker_client.go:188       container is ready      {"id": "dcaeabe514afd96bebd6fd8fe0cd83d36fb725bc9bf01f911e6158d926eb141a", "work-dir": "/tmp/texd-483222946"}
2022-03-17T13:40:18.364+0100    DEBUG   tex/document.go:298     fetching result {"request-id": "0767bf4c-a8ee-4832-836f-df6a7d096224"}

My primary issues are:

• The timestamp should be removed or at least shortened to "seconds since startup" (like logrus).
• There's no color. I like color in dev mode.
• Fields are not aligned.

These issues can be easily addressed using either a custom zapcore.EncoderConfig, or by providing a custom encoder (zap.RegisterEncoder). More importantly, this can be addressed later.

[corny]

Let's switch to the log/slog package for structured logging.

[dmke]

I don't object, but need to find time to do the conversion. Fancy a PR?