1. A system, comprising: a plurality of computing devices configured to implement resources of a provider network, wherein the resources are configurable for network access from client devices via one or more provider network endpoints; and one or more computing devices configured to: collect, using a first set of one or more programmatic interfaces, network service offering metadata from a plurality of connectivity providers; wherein one or more of the plurality of connectivity providers control physical infrastructure configured to provide at least a portion of a network path between a client network and at least one of the one or more provider network endpoints; receive a connectivity query from a client via a first connection, wherein the connectivity query indicates one or more client connectivity parameters for a desired dedicated private network path between the client network and the at least one of the one or more provider network endpoints, wherein the first connection is established at least in part via one or more public network paths; in response to the connectivity query indicating the one or more client connectivity parameters, identify, using at least a portion of the network service offering metadata, a collection of one or more connectivity providers of the plurality of connectivity providers for providing physical connectivity for the desired dedicated private network path from the client network and a provider network endpoint in accordance with the one or more client connectivity parameters; provide to the client a list of connectivity providers from the collection of connectivity providers for providing the physical connectivity from the client network to the provider network endpoint of the one or more provider network endpoints in accordance with the one or more client connectivity parameters; receive a connectivity establishment request from the client, wherein the connectivity establishment request indicates a selection of one or more of the plurality of connectivity providers selected to implement the desired dedicated private network path from the client network to the at least one of the one or more provider network endpoints; and in response to the connectivity establishment request indicating the one or more selected connectivity providers of the collection, initiate, using a second set of one or more programmatic interfaces, an activation of the desired dedicated private network path over a physical network connection from the client network to a selected provider network endpoint of the one or more provider network endpoints via the one or more selected connectivity providers, wherein traffic that is not generated on behalf of the client is not transmitted over the physical network connection from the client network to the selected provider network endpoint of the dedicated private network path, wherein operation of at least one physical network link used for the physical network connection is implemented by a particular connectivity provider of the one or more selected connectivity providers that controls physical infrastructure configured to provide the physical network link to the selected provider network endpoint.