Expose whether a DB connection came from the connection pool

It would be useful for userland Hack code in Slack to determine
whether a connection obtained via an HHVM async_mysql connection pool
was recycled from the pool or freshly opened, to allow more granular
instrumentation than what pool-level stats currently provide.
See facebook#8013 for the full rationale.

The groundwork for this on the Squangle side has already been laid
in D92081429 née facebook/squangle#20.
Add methods to `AsyncMysqlConnection` that make use of the newly added
accessors to expose this information to Hack.

hphp/test/slow/ext_async_mysql/ doesn't seem to currently have tests
that interact with a MySQL server so I didn't include tests in this patch.
I can add some if needed.

Closes facebook#8013.

Co-authored-by: Henry Robinson <hrobinson@slack-corp.com>

Author: mszabo-wikia

hphp/hack/hhi/stdlib/builtins_async_mysql.hhi

@@ -218,6 +218,8 @@ namespace {
     public function getSslCertSan(): Vector<string> {}
     public function getSslCertExtensions(): Vector<string> {}
     public function isSslCertValidationEnforced(): bool {}
+    public function wasFromPoolHit(): bool {}
+    public function wasReusedWithChangeUser(): bool {}
   }
 
   abstract class AsyncMysqlResult {

hphp/runtime/ext/async_mysql/ext_async_mysql.cpp

@@ -1208,6 +1208,18 @@ static bool HHVM_METHOD(AsyncMysqlConnection, isSSL) {
   return data->m_conn->isSSL();
 }
 
+static bool HHVM_METHOD(AsyncMysqlConnection, wasFromPoolHit) {
+  auto* data = Native::data<AsyncMysqlConnection>(this_);
+  data->verifyValidConnection();
+  return data->m_conn->wasFromPoolHit();
+}
+
+static bool HHVM_METHOD(AsyncMysqlConnection, wasReusedWithChangeUser) {
+  auto* data = Native::data<AsyncMysqlConnection>(this_);
+  data->verifyValidConnection();
+  return data->m_conn->wasReusedWithChangeUser();
+}
+
 static int64_t HHVM_METHOD(AsyncMysqlConnection, warningCount) {
   auto* data = Native::data<AsyncMysqlConnection>(this_);
 
@@ -2264,6 +2276,8 @@ static struct AsyncMysqlExtension final : Extension {
     HHVM_ME(AsyncMysqlConnection, port);
     HHVM_ME(AsyncMysqlConnection, setReusable);
     HHVM_ME(AsyncMysqlConnection, isReusable);
+    HHVM_ME(AsyncMysqlConnection, wasFromPoolHit);
+    HHVM_ME(AsyncMysqlConnection, wasReusedWithChangeUser);
     HHVM_ME(AsyncMysqlConnection, connectResult);
     HHVM_ME(AsyncMysqlConnection, lastActivityTime);
     HHVM_ME(AsyncMysqlConnection, getSslCertCn);

hphp/runtime/ext/async_mysql/ext_async_mysql.php

@@ -664,6 +664,20 @@ public function getSslCertExtensions(): Vector<string>;
    */
   <<__Native>>
   public function isSslCertValidationEnforced(): bool;
+
+  /**
+   * Whether this connection was newly opened or recycled from a connection pool.
+   * @return - "true" if the pool was recycled from a connection pool, "false" otherwise.
+   */
+  <<__Native>>
+  public function wasFromPoolHit(): bool;
+
+  /**
+   * Whether this connection was recycled from a connection pool with changed credentials.
+   * @return - "true" if the pool was recycled from a connection pool with a credential change, "false" otherwise.
+   */
+  <<__Native>>
+  public function wasReusedWithChangeUser(): bool;
 }
 
 /**