client: avoid spamming telemetry on initial sync (#5651)

substrate/Cargo.lock

@@ -5905,6 +5905,7 @@ dependencies = [
  "log",
  "parity-scale-codec",
  "parking_lot 0.10.2",
+ "rand 0.7.3",
  "sc-block-builder",
  "sc-client-api",
  "sc-executor",

substrate/client/Cargo.toml

@@ -28,6 +28,7 @@ sp-keyring = { version = "2.0.0-dev", path = "../primitives/keyring" }
 kvdb = "0.5.0"
 log = { version = "0.4.8" }
 parking_lot = "0.10.0"
+rand = "0.7.3"
 sp-core = { version = "2.0.0-dev", path = "../primitives/core" }
 sp-std = { version = "2.0.0-dev", path = "../primitives/std" }
 sp-version = { version = "2.0.0-dev", path = "../primitives/version" }

substrate/client/src/client.rs

@@ -613,11 +613,20 @@ impl<B, E, Block, RA> Client<B, E, Block, RA> where
 
 		if let Ok(ImportResult::Imported(ref aux)) = result {
 			if aux.is_new_best {
-				telemetry!(SUBSTRATE_INFO; "block.import";
-					"height" => height,
-					"best" => ?hash,
-					"origin" => ?origin
-				);
+				use rand::Rng;
+				
+				// don't send telemetry block import events during initial sync for every
+				// block to avoid spamming the telemetry server, these events will be randomly
+				// sent at a rate of 1/10.
+				if origin != BlockOrigin::NetworkInitialSync ||
+					rand::thread_rng().gen_bool(0.1)
+				{
+					telemetry!(SUBSTRATE_INFO; "block.import";
+						"height" => height,
+						"best" => ?hash,
+						"origin" => ?origin
+					);
+				}
 			}
 		}