Add metric for the space taken of the symbol tables

[brian-brazil]

If a user has way too many label values this could get large, so having a metric indicating the space taken would be useful.

[gouthamve]

Should this be a prometheus metric or should it be part of the tsdb/promtool cli?

[brian-brazil]

A metric of the TSDB itself, as Prometheus has no knowledge of this.

[sipian]

If there is no one working on this, I would like to take this issue up.

[sipian]

In index/index.go there is a function, func (r *Reader) SymbolTable() map[uint32]string

Based on my understanding, to get the symbol table size we are supposed to call this function and get the total size of the map[uint32]string. For map total size, I found this reference

Is this the correct direction in solving this issue?

[gouthamve]

As I spoke offline, this is about the size of the symbol table on disk rather than the in-mem version.

[brian-brazil]

Either in-memory or disk works, they're both basically the same size.