The Metropolitan Opera, plagued by labor disputes, opened last Friday night, nearly three months late. However, in presenting the first New York performance of the complete score of Alban Berg's "Lulu," the Met gave New York what is probably the most significant musical event of the year.

The history of the opera is as fascinating as its undeniably sordid, decadent story. At the time of Berg's death in 1935, the composer had finished the entire three-act score, but had not completed orchestrating the final act. After World War II, Berg's widow, Helene, began a long series of refusals to various musicians and publishers who implored her to permit the finishing of the great work.

Often, having given consent on the previous day, the Widow Berg would withdraw permission, saying that her late husband had visited her during the night. She invariably insisted, "Alban says 'No.'"

Even before her death in 1976, however, the Viennese composer Friedrich Cerha, working under a legal but secret agreement with the publisher, had completed the scoring, and in 1978 the entire work was finally given its premiere at the Paris Opera under the baton of Pierre Boulez. Last summer "Lulu" was heard for the first time in this country at the Santa Fe Opera. On Friday night, James Levine conducted a transcendental performance, and next Saturday night the opera will be seen and heard live on television from the Met.

The production is, in every way, a triumph. It places "Lulu" before us with such a wealth of new musical material, all of which provides both dramatic and musical balance to the truncated versions previously heard, that there can be no question of the place of permanence this opera deserves and will undoubtedly soon occupy as it moves through the world's foremost opera theaters.

Based on Wedekind's plays "Earth Spirit" and "Pandora's Box," Berg's music is a magnificent distillation of that final romantic era that culminated in Wagner's "Tristan," which it constantly recalls; that spilled over into the symphonies of Gustav Mahler, which are continually echoed in the impassioned love music; and was finally dissipated in the haunting sound of saxophones throbbing away in numberless postwar cafes.

The Metropolitan's orchestra played superbly for Levine, providing sensuous sonorities with elegant style. John Dexter's production, designed in 1976 for the Met's first performances of the two then-available acts, has been expanded to include the stunning Parisian gambling house scene that opens the now-completed third act. Perfect touches of color appear: Lulu's dressing room in act one is a vision of purple with fringe that aptly recalls the late 1880s. Mirrors are dramatically employed, and elaborate props remind the audience that the whole opera is, as the prologue announces, a circus tent filled with wild beasts.

The cast is a procession of superbly realized protraits too long to enumerate. Teresa Stratas sings and acts the title role consummately, having it completely in command, though its stratospheric range frequently threatens to tear her voice to shreds. Unfortunately it is that kind of role. Evelyn Lear, who remains the greatest Lulu to date, admitted after relinquishing the part that it took years off her voice. Stratas also makes telling use of an expressive speaking voice, in a depiction of Lulu as an amoral sex object that is unceasingly compelling.

Out of the long cast, highest praise must go to Frank Mazura in the doubling role of Dr. Schoen and Jack the Ripper. As the former, he is murdered by Lulu; as the latter, he is her murderer. Lear, as the Countess Geschwitz, in love with Lulu, contributed some of the finest singing and acting. Lenus Carlson as Animal Tamer and Acrobat is dazzling in action and singing. Andrew Foldi makes old asthmatic Schigolch an unforgettable portrait in the huge canvas, while Kenneth Riegel's Alwa is fluently sung and admirably acted. Gil Wechsler's lighting added a hundred subtle psychological touches that greatly enhanced the sets and costumes of Jocelyn Herbert. With this magnificent production, the Metropolitan has added another page of brightest luster to its long history.