The weather reports painted a gloomy picture, but Michael Conforto spent his Sunday morning hoping for the rain to stay away. His opportunity finally had come.

For the Mets’ homegrown hitting star, the final remnant of training wheels has been action against tough lefties. But even with Giants ace Madison Bumgarner on the mound, manager Terry Collins couldn’t resist keeping Conforto’s name on the lineup card.

“If I stick to my game plan against these guys, I can have success,” Conforto said before the Mets’ 6-1 loss to the Giants. “They’re very, very good. They’re the best in the world. But they’re not unhittable. Lefties get hits off of them throughout the year, and I think I kind of take the attitude of ‘why not me?’ I don’t see any reason why not.”

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For one day, Conforto got a bitter taste of the torture that Bumgarner has inflicted upon even the most experienced lefthanded hitters.

Until Sunday, Conforto had never seen for himself Bumgarner’s tricky cross-body delivery, one that makes baseballs appear as if they’re coming from behind a lefthanded hitter’s back. He had never seen Bumgarner’s command of a slider and fastball that appear almost indistinguishable out of his hand.

The results were predictable. In three at-bats against Bumgarner, Conforto struck out twice and flied out. He went 0-for-5 for the day, ending a career-high streak of reaching base in 17 consecutive games.

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“As a competitor, you want to be out there against the best guys, and he’s a guy that I definitely wanted to see how I matched up,” Conforto said. “Today, he got the best of me. That obviously doesn’t sit well with me.”

Indeed, it was the kind of day that was bound to happen for the Mets (15-8).

For the first time this season, fireballing righty Noah Syndergaard appeared merely human. Not only did he allow four runs in 5 2⁄3 innings, but the Giants punished him on the basepaths, exposing what has been a persistent problem with keeping runners honest. And the Mets watched their season-best eight-game winning streak came to an end.

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However, on an otherwise forgettable afternoon, the Mets began the important process of learning how their most promising young hitter will handle facing the game’s toughest southpaws.

Until Sunday, the only person consistently capable of keeping Conforto off the bases of late has been Collins, who has insisted upon shielding the young talent against lefties. But the manager finally broke from policy.

Said Collins: “He’s just swinging too good.”

Conforto began the day among the National League leaders in average (.365), on-base percentage (.442) and slugging (.676). So with Bumgarner on the hill, Curtis Granderson wound up on the bench.

Conforto, 23, has been a quick study since his arrival in the big leagues last July from Double-A Binghamton. He already has learned to obey a fine line. While he has respected his foes, he’s stopped short of regarding them as deities.

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For instance, Bumgarner had his number. And while Conforto acknowledged as much, he also noted, “I had multiple pitches I could have [driven].”

Despite the results, Collins acknowledged what Conforto’s exposure to tough lefties could mean in the future.

“I’m glad he faced [Bumgarner],” Collins said. “Because when it comes to crunch time, that might be a guy he’s going to have to face again.”

Plenty of tough southpaws will stand between the Mets and their goal of winning the World Series.

On an upcoming road trip, the Mets could see one of them in Dodgers star Clayton Kershaw. Before the end of May, they could see another in White Sox ace Chris Sale. Conforto wants a piece of them, too.

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“I want to be in the lineup against Kershaw, I want to be in the lineup if we face Chris Sale, or for any of those guys,” he said. “I think it can only help me grow as a player to be in there against those guys.”