When streamingForwardTeamHeadersTempo is enabled, gRPC metadata is forwarded
alongside datasource-configured HTTP headers. gRPC lowercases metadata keys,
so an incoming x-scope-orgid would land in the merged map next to the
datasource's canonical X-Scope-Orgid. On the wire both serialize to the same
case-insensitive name and Go's randomised map iteration can deliver the wrong
value, surfacing as Tempo's "no org id" error.
Drop team headers whose key collides case-insensitively with a
datasource-configured header so datasource settings always take precedence.
Fixes#123923