Use tenant ID from incoming gRPC meta for instance caching

[wbrowne]

What this PR does / why we need it:
Adds a new tenant package which is responsible for reading tenant information for each incoming plugin request. This information will be attached to the Go context, to be used as supplementary info for instance management caching.

This approach is documented as option 2 in this doc.

Special notes for your reviewer:
The InstanceManager API requires a new context.Context argument for the relevant information to be propagated, which is the primary breaking change. This has an effect on core plugins and a number of community plugins.

Breakdown of the changes:

# github.com/grafana/grafana-plugin-sdk-go/backend/instancemgmt
## incompatible changes
InstanceManager.Do: changed from func(github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext, InstanceCallbackFunc) error to func(context.Context, github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext, InstanceCallbackFunc) error
InstanceManager.Get: changed from func(github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext) (Instance, error) to func(context.Context, github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext) (Instance, error)
InstanceProvider.GetKey: changed from func(github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext) (interface{}, error) to func(context.Context, github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext) (interface{}, error)
InstanceProvider.NeedsUpdate: changed from func(github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext, CachedInstance) bool to func(context.Context, github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext, CachedInstance) bool
InstanceProvider.NewInstance: changed from func(github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext) (Instance, error) to func(context.Context, github.com/grafana/grafana-plugin-sdk-go/backend.PluginContext) (Instance, error)

# github.com/grafana/grafana-plugin-sdk-go/backend/tenant
## compatible changes
package added

A follow up will come to likely remove instance management (where possible) from the public API, in favour of the auto instance management functionality which is already used as part of the primary entrypoints for both apps and data sources

[marefr]

LGTM

[wbrowne]

Side note @bergquist, had you considered what (if anything) would change about how we handle metrics based on tenant info https://github.com/grafana/grafana-plugin-sdk-go/blob/main/backend/diagnostics_adapter.go#L25

[xnyo]

LGTM, amazing work 🚀!

Regarding the discussion around tenant id in context vs plugin context, I don't have any strong opinions, but I definitely agree about removing manual instance management from the public api in favor of automatic instance management.

Also worth mentioning that PluginContext is a GRPC message, and adding tenant id over there would require changing the protobuf definition. Since we're not 100% sure yet how this deployment model will look like at the end, I personally think it's safer to have it in the grpc metadata for now, as that's much easier to remove/deprecate compared to protobuf.

[andresmgot]

okay, let's go ahead with this if everyone agrees