*: cherry pick some improvements to release 3.0 (tikv#1815)

* *: reset metrics after the leader steps down (tikv#1790)

Signed-off-by: Ryan Leung <rleungx@gmail.com>

* *: improve label metrics (tikv#1753)

Signed-off-by: Ryan Leung <rleungx@gmail.com>

add config-check flag for pd-server (tikv#1695) (tikv#1725)

Signed-off-by: cwen0 <cwenyin0@gmail.com>

add config-check flag for pd-server (tikv#1695) (tikv#1711)

Signed-off-by: cwen0 <cwenyin0@gmail.com>

*: bump to v2.1.15 (tikv#1644)

Signed-off-by: youjiali1995 <zlwgx1023@gmail.com>

Bump version to 3.0.1 (tikv#1640)

Signed-off-by: Shafreeck Sea <shafreeck@gmail.com>

changelog: add v2.1.14 changelog (tikv#1618)

Signed-off-by: disksing <i@disksing.com>