refactor: Make prometheus features push and process optional to get rid of openssl (#433)

* refactor: Make prometheus deps optional

Signed-off-by: Xuanwo <github@xuanwo.io>

* Remove push and process

Signed-off-by: Xuanwo <github@xuanwo.io>

---------

Signed-off-by: Xuanwo <github@xuanwo.io>

Author: Xuanwo

Cargo.toml

@@ -9,7 +9,8 @@ description = "The Rust language implementation of TiKV client."
 edition = "2021"
 
 [features]
-default = ["prometheus/process"]
+default = ["prometheus"]
+prometheus = ["prometheus/push", "prometheus/process"]
 # Enable integration tests with a running TiKV and PD instance.
 # Use $PD_ADDRS, comma separated, to set the addresses the tests use.
 integration-tests = []
@@ -33,7 +34,7 @@ futures = { version = "0.3" }
 lazy_static = "1"
 log = "0.4"
 pin-project = "1"
-prometheus = { version = "0.13", features = ["push"], default-features = false }
+prometheus = { version = "0.13", default-features = false }
 prost = "0.12"
 rand = "0.8"
 regex = "1"