The water agency that supplies drinking water to Los Angeles agreed Tuesday to contribute $1.5 million toward the planning of Sites Reservoir in the Sacramento Valley, giving the agency a toehold in a potentially valuable storage project. The board of the Metropolitan Water District of Southern California, as expected, approved the expenditure by a unanimous vote. If the agency eventually decides to contribute to the construction project itself, it would entitle Metropolitan to control up to 50,000 acre-feet of storage in the reservoir. Sites, to be built on the Glenn-Colusa county line, would store up to 1.8 million acre-feet.