Add grandpa telemetry (+ other minor improvements) (#1997)

* Support skipping the interactive purge prompt

Skip it via `cargo run -- purge-chain -y`.

* Fix typos

* Add Grandpa telemetry

* Address comments

* Revert unnecessary version bump

* Version bump to make CI run

* Remove unnecessary cast

* Do not bump version

substrate/core/basic-authorship/src/basic_authorship.rs

@@ -37,6 +37,7 @@ use runtime_primitives::generic::BlockId;
 use runtime_primitives::ApplyError;
 use transaction_pool::txpool::{self, Pool as TransactionPool};
 use inherents::InherentData;
+use substrate_telemetry::{telemetry, CONSENSUS_INFO};
 
 /// Build new blocks.
 pub trait BlockBuilder<Block: BlockT> {
@@ -252,6 +253,10 @@ impl<Block, C, A> Proposer<Block, C, A>	where
 				.collect::<Vec<_>>()
 				.join(", ")
 		);
+		telemetry!(CONSENSUS_INFO; "prepared_block_for_proposing";
+			"number" => ?block.header().number(),
+			"hash" => ?<<C as AuthoringApi>::Block as BlockT>::Hash::from(block.header().hash()),
+		);
 
 		let substrate_block = Decode::decode(&mut block.encode().as_slice())
 			.expect("blocks are defined to serialize to substrate blocks correctly; qed");