add unit test for v3 metrics for all its exposed endpoints
Bonus:
- support OpenMetrics encoding
- adds boot time for prometheus
- continueOnError is better to serve as
much metrics as possible.
```
curl http://localhost:9000/minio/metrics/v3/cluster/usage/buckets
```
Did not work as documented, due to the fact that there was a typo
in the bucket usage metrics registration group. This endpoint is
a cluster endpoint and does not require any `buckets` argument.
Per-bucket metrics endpoints always start with /bucket and the bucket
name is appended to the path. e.g. if the collector path is /bucket/api,
the endpoint for the bucket "mybucket" would be
/minio/metrics/v3/bucket/api/mybucket
Change the existing bucket api endpoint accordingly from /api/bucket to
/bucket/api
Create new code paths for multiple subsystems in the code. This will
make maintaing this easier later.
Also introduce bugLogIf() for errors that should not happen in the first
place.
Metrics v3 is mainly a reorganization of metrics into smaller groups of
metrics and the removal of internal aggregation of metrics received from
peer nodes in a MinIO cluster.
This change adds the endpoint `/minio/metrics/v3` as the top-level metrics
endpoint and under this, various sub-endpoints are implemented. These
are currently documented in `docs/metrics/v3.md`
The handler will serve metrics at any path
`/minio/metrics/v3/PATH`, as follows:
when PATH is a sub-endpoint listed above => serves the group of
metrics under that path; or when PATH is a (non-empty) parent
directory of the sub-endpoints listed above => serves metrics
from each child sub-endpoint of PATH. otherwise, returns a no
resource found error
All available metrics are listed in the `docs/metrics/v3.md`. More will
be added subsequently.
