Improved metrics

[adria0]

Added db metrics

To prevent a larger refactor, is decided just to keep the KeyValueDB name that comes from kvdb crate, but redefining it in the local ethcore_db

• All uses of KeyValueDB now uses the internal crate ethcore_db instead the external crate kvdb
• The ethcore_db::KeyValueDB is defined as pub trait KeyValueDB: kvdb::KeyValueDB + PrometheusMetrics {}
• There are implemented two wrappers that implements the ethcore_db::KeyValueDB trait
  • DatabaseWithMetrics that wraps Database and records its metrics (disk rocksdb)
  • InMemoryWithMetrics that wraps InMemory (memory db)

Metrics recorded are:

• kvdb_bytes_read
• kvdb_bytes_written
• kvdb_reads
• kvdb_writes

note1: DB Row iterators are rarely used, so is not implemented doing an increment on kv_xxx_read counter.
note2: at this moment kvdb_reads metrics matches kvdb_writes, we already detected this when implementing metrics first time in 3.0.0 taking

Added metrics prefix

Now is possible to specify the metric prefix by using --metrics-prefix=[prefix] command line (requested by @denisgranha)

• A new PrometheusRegistry with register_ methods is used instead of using prometheus_ functions with prometheus::Registry
• PrometheusMetrics trait changed to use PrometheusRegistry

[jomuel]

The build failure seems to be unrelated?

[rakita]

lgtm