db: add UserKeyCategories

[RaduBerinde]

Add a way to configure Pebble with an informative partitioning of the
key space. Each partition is a named key category; we use these for
pprof labels.

We currently set these pprof labels for compactions. In the future, we
will allow turning on labels in the read/write paths for specific
investigations.

Sample CRDB configuration code:

var userKeyCategories = pebble.MakeUserKeyCategories(
  EngineComparer.Compare,
  []pebble.UserKeyCategory{
    {Name: "local-1", UpperBound: toEngineKey(keys.LocalRangeIDPrefix.AsRawKey())},
    {Name: "rangeid", UpperBound: toEngineKey(keys.LocalRangeIDPrefix.AsRawKey().PrefixEnd())},
    {Name: "local-2", UpperBound: toEngineKey(keys.LocalRangePrefix)},
    {Name: "range", UpperBound: toEngineKey(keys.LocalRangePrefix.PrefixEnd())},
    {Name: "local-3", UpperBound: toEngineKey(keys.LocalRangeLockTablePrefix)},
    {Name: "lock", UpperBound: toEngineKey(keys.LocalRangeLockTablePrefix.PrefixEnd())},
    {Name: "local-4", UpperBound: toEngineKey(keys.LocalPrefix.PrefixEnd())},
    {Name: "meta", UpperBound: toEngineKey(keys.MetaMax)},
    {Name: "system", UpperBound: toEngineKey(keys.SystemMax)},
    {Name: "tenant"},
  }...,
)

[cockroach-teamcity]

This change is 

[RaduBerinde]

Sample flame graph with -tagroot key-type:

[jbowens]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @itsbilal and @RaduBerinde)

---

options.go line 2209 at r1 (raw file):

// UserKeyCategories describes a partitioning of the user key space. Each
// partition is a category with a name, which is used for informative purposes
// (like pprof labels).

maybe add something explicitly emphasizing that these partitions are not physically materialized

---

options.go line 2243 at r1 (raw file):

	}
	if categories[n-1].UpperBound != nil {
		panic("last category UperBound must be nil")

nit: UpperBound

[RaduBerinde]

TFTR!

Reviewable status: 2 of 3 files reviewed, 1 unresolved discussion (waiting on @itsbilal and @jbowens)

---

options.go line 2209 at r1 (raw file):

Previously, jbowens (Jackson Owens) wrote…

maybe add something explicitly emphasizing that these partitions are not physically materialized

Done.