* Improve error logging for missing config and QL dir
Currently, when Prometheus can't open its config file or the query
logging dir under the data dir, it only logs what it has been given
default or commandline/config. Depending on the environment this can be
less than helpful, since the working directory may be unclear to the
user. I have specifically kept the existing error messages as intact as
possible to a) still log the parameter as given and b) cause as little
disruption for log-parsers/-analyzers as possible.
So in case of the config file or the data dir being non-absolute paths,
I use os.GetWd to find the working dir and assemble an absolute path for
error logging purposes. If GetWd fails, we just log "unknown", as
recovering from an error there would be very complex measure, likely not
worth the code/effort.
Example errors:
```
$ ./prometheus
ts=2022-02-06T16:00:53.034Z caller=main.go:445 level=error msg="Error loading config (--config.file=prometheus.yml)" fullpath=/home/klausman/src/prometheus/prometheus.yml err="open prometheus.yml: no such file or directory"
$ touch prometheus.yml
$ ./prometheus
[...]
ts=2022-02-06T16:01:00.992Z caller=query_logger.go:99 level=error component=activeQueryTracker msg="Error opening query log file" file=data/queries.active fullpath=/home/klausman/src/prometheus/data/queries.active err="open data/queries.active: permission denied"
panic: Unable to create mmap-ed active query log
[...]
$
```
Signed-off-by: Tobias Klausmann <klausman@schwarzvogel.de>
* Replace our own logic with just using filepath.Abs()
Signed-off-by: Tobias Klausmann <klausman@schwarzvogel.de>
* Further simplification
Signed-off-by: Tobias Klausmann <klausman@schwarzvogel.de>
* Review edits
Signed-off-by: Tobias Klausmann <klausman@schwarzvogel.de>
* Review edits
Signed-off-by: Tobias Klausmann <klausman@schwarzvogel.de>
* Review edits
Signed-off-by: Tobias Klausmann <klausman@schwarzvogel.de>
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