Rename lru cache metrics with a unified "dgraph_lru" prefix.

[danielmai]

This change is 

[manishrjain]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @danielmai)

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wiki/content/deploy/index.md, line 1430 at r1 (raw file):

The LRU cache metrics let you track on how well the posting list cache is being used.

You can track `dgraph_lru_capacity_bytes`, `dgraph_lru_evicted_total`, and `dgraph_max_list_bytes` (see the [Data Metrics]({{< relref "#data-metrics" >}})) to determine if the cache size should be adjusted. A high number of evictions can indicate a large posting list that repeatedly is inserted and evicted from cache due to insufficient sizing. The LRU cache size can be tuned with the option `--lru_mb`.

100/80 chars.

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x/metrics.go, line 69 at r1 (raw file):

	BytesRead = expvar.NewInt("dgraph_read_bytes_total")
	BytesWrite = expvar.NewInt("dgraph_written_bytes_total")
	EvictedPls = expvar.NewInt("dgraph_lru_evicted_total")

LcacheEvicts

[danielmai]

Reviewable status: 0 of 3 files reviewed, 2 unresolved discussions (waiting on @manishrjain)

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wiki/content/deploy/index.md, line 1430 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

100/80 chars.

Done.

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x/metrics.go, line 69 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

LcacheEvicts

Done.